compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

   1 use super::{
   2     DefIdOrName, Obligation, ObligationCause, ObligationCauseCode, PredicateObligation,
   3     SelectionContext,
   4 };
   5
   6 use crate::autoderef::Autoderef;
   7 use crate::infer::InferCtxt;
   8 use crate::traits::normalize_to;
   9
  10 use hir::def::CtorOf;
  11 use hir::HirId;
  12 use rustc_data_structures::fx::FxHashSet;
  13 use rustc_data_structures::stack::ensure_sufficient_stack;
  14 use rustc_errors::{
  15     error_code, pluralize, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder,
  16     ErrorGuaranteed, MultiSpan, Style,
  17 };
  18 use rustc_hir as hir;
  19 use rustc_hir::def::DefKind;
  20 use rustc_hir::def_id::DefId;
  21 use rustc_hir::intravisit::Visitor;
  22 use rustc_hir::lang_items::LangItem;
  23 use rustc_hir::{AsyncGeneratorKind, GeneratorKind, Node};
  24 use rustc_infer::infer::error_reporting::TypeErrCtxt;
  25 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  26 use rustc_infer::infer::LateBoundRegionConversionTime;
  27 use rustc_middle::hir::map;
  28 use rustc_middle::ty::{
  29     self, suggest_arbitrary_trait_bound, suggest_constraining_type_param, AdtKind, DefIdTree,
  30     GeneratorDiagnosticData, GeneratorInteriorTypeCause, Infer, InferTy, IsSuggestable,
  31     ToPredicate, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitable,
  32 };
  33 use rustc_middle::ty::{TypeAndMut, TypeckResults};
  34 use rustc_span::symbol::{sym, Ident, Symbol};
  35 use rustc_span::{BytePos, DesugaringKind, ExpnKind, Span, DUMMY_SP};
  36 use rustc_target::spec::abi;
  37 use std::fmt;
  38
  39 use super::InferCtxtPrivExt;
  40 use crate::infer::InferCtxtExt as _;
  41 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  42 use rustc_middle::ty::print::with_no_trimmed_paths;
  43
  44 #[derive(Debug)]
  45 pub enum GeneratorInteriorOrUpvar {
  46     // span of interior type
  47     Interior(Span, Option<(Option<Span>, Span, Option<hir::HirId>, Option<Span>)>),
  48     // span of upvar
  49     Upvar(Span),
  50 }
  51
  52 // This type provides a uniform interface to retrieve data on generators, whether it originated from
  53 // the local crate being compiled or from a foreign crate.
  54 #[derive(Debug)]
  55 pub enum GeneratorData<'tcx, 'a> {
  56     Local(&'a TypeckResults<'tcx>),
  57     Foreign(&'tcx GeneratorDiagnosticData<'tcx>),
  58 }
  59
  60 impl<'tcx, 'a> GeneratorData<'tcx, 'a> {
  61     // Try to get information about variables captured by the generator that matches a type we are
  62     // looking for with `ty_matches` function. We uses it to find upvar which causes a failure to
  63     // meet an obligation
  64     fn try_get_upvar_span<F>(
  65         &self,
  66         infer_context: &InferCtxt<'tcx>,
  67         generator_did: DefId,
  68         ty_matches: F,
  69     ) -> Option<GeneratorInteriorOrUpvar>
  70     where
  71         F: Fn(ty::Binder<'tcx, Ty<'tcx>>) -> bool,
  72     {
  73         match self {
  74             GeneratorData::Local(typeck_results) => {
  75                 infer_context.tcx.upvars_mentioned(generator_did).and_then(|upvars| {
  76                     upvars.iter().find_map(|(upvar_id, upvar)| {
  77                         let upvar_ty = typeck_results.node_type(*upvar_id);
  78                         let upvar_ty = infer_context.resolve_vars_if_possible(upvar_ty);
  79                         if ty_matches(ty::Binder::dummy(upvar_ty)) {
  80                             Some(GeneratorInteriorOrUpvar::Upvar(upvar.span))
  81                         } else {
  82                             None
  83                         }
  84                     })
  85                 })
  86             }
  87             GeneratorData::Foreign(_) => None,
  88         }
  89     }
  90
  91     // Try to get the span of a type being awaited on that matches the type we are looking with the
  92     // `ty_matches` function. We uses it to find awaited type which causes a failure to meet an
  93     // obligation
  94     fn get_from_await_ty<F>(
  95         &self,
  96         visitor: AwaitsVisitor,
  97         hir: map::Map<'tcx>,
  98         ty_matches: F,
  99     ) -> Option<Span>
 100     where
 101         F: Fn(ty::Binder<'tcx, Ty<'tcx>>) -> bool,
 102     {
 103         match self {
 104             GeneratorData::Local(typeck_results) => visitor
 105                 .awaits
 106                 .into_iter()
 107                 .map(|id| hir.expect_expr(id))
 108                 .find(|await_expr| {
 109                     ty_matches(ty::Binder::dummy(typeck_results.expr_ty_adjusted(&await_expr)))
 110                 })
 111                 .map(|expr| expr.span),
 112             GeneratorData::Foreign(generator_diagnostic_data) => visitor
 113                 .awaits
 114                 .into_iter()
 115                 .map(|id| hir.expect_expr(id))
 116                 .find(|await_expr| {
 117                     ty_matches(ty::Binder::dummy(
 118                         generator_diagnostic_data
 119                             .adjustments
 120                             .get(&await_expr.hir_id.local_id)
 121                             .map_or::<&[ty::adjustment::Adjustment<'tcx>], _>(&[], |a| &a[..])
 122                             .last()
 123                             .map_or_else::<Ty<'tcx>, _, _>(
 124                                 || {
 125                                     generator_diagnostic_data
 126                                         .nodes_types
 127                                         .get(&await_expr.hir_id.local_id)
 128                                         .cloned()
 129                                         .unwrap_or_else(|| {
 130                                             bug!(
 131                                                 "node_type: no type for node `{}`",
 132                                                 ty::tls::with(|tcx| tcx
 133                                                     .hir()
 134                                                     .node_to_string(await_expr.hir_id))
 135                                             )
 136                                         })
 137                                 },
 138                                 |adj| adj.target,
 139                             ),
 140                     ))
 141                 })
 142                 .map(|expr| expr.span),
 143         }
 144     }
 145
 146     /// Get the type, expression, span and optional scope span of all types
 147     /// that are live across the yield of this generator
 148     fn get_generator_interior_types(
 149         &self,
 150     ) -> ty::Binder<'tcx, &[GeneratorInteriorTypeCause<'tcx>]> {
 151         match self {
 152             GeneratorData::Local(typeck_result) => {
 153                 typeck_result.generator_interior_types.as_deref()
 154             }
 155             GeneratorData::Foreign(generator_diagnostic_data) => {
 156                 generator_diagnostic_data.generator_interior_types.as_deref()
 157             }
 158         }
 159     }
 160
 161     // Used to get the source of the data, note we don't have as much information for generators
 162     // originated from foreign crates
 163     fn is_foreign(&self) -> bool {
 164         match self {
 165             GeneratorData::Local(_) => false,
 166             GeneratorData::Foreign(_) => true,
 167         }
 168     }
 169 }
 170
 171 // This trait is public to expose the diagnostics methods to clippy.
 172 pub trait TypeErrCtxtExt<'tcx> {
 173     fn suggest_restricting_param_bound(
 174         &self,
 175         err: &mut Diagnostic,
 176         trait_pred: ty::PolyTraitPredicate<'tcx>,
 177         associated_item: Option<(&'static str, Ty<'tcx>)>,
 178         body_id: hir::HirId,
 179     );
 180
 181     fn suggest_dereferences(
 182         &self,
 183         obligation: &PredicateObligation<'tcx>,
 184         err: &mut Diagnostic,
 185         trait_pred: ty::PolyTraitPredicate<'tcx>,
 186     ) -> bool;
 187
 188     fn get_closure_name(&self, def_id: DefId, err: &mut Diagnostic, msg: &str) -> Option<Symbol>;
 189
 190     fn suggest_fn_call(
 191         &self,
 192         obligation: &PredicateObligation<'tcx>,
 193         err: &mut Diagnostic,
 194         trait_pred: ty::PolyTraitPredicate<'tcx>,
 195     ) -> bool;
 196
 197     fn suggest_add_reference_to_arg(
 198         &self,
 199         obligation: &PredicateObligation<'tcx>,
 200         err: &mut Diagnostic,
 201         trait_pred: ty::PolyTraitPredicate<'tcx>,
 202         has_custom_message: bool,
 203     ) -> bool;
 204
 205     fn suggest_borrowing_for_object_cast(
 206         &self,
 207         err: &mut Diagnostic,
 208         obligation: &PredicateObligation<'tcx>,
 209         self_ty: Ty<'tcx>,
 210         object_ty: Ty<'tcx>,
 211     );
 212
 213     fn suggest_remove_reference(
 214         &self,
 215         obligation: &PredicateObligation<'tcx>,
 216         err: &mut Diagnostic,
 217         trait_pred: ty::PolyTraitPredicate<'tcx>,
 218     ) -> bool;
 219
 220     fn suggest_remove_await(&self, obligation: &PredicateObligation<'tcx>, err: &mut Diagnostic);
 221
 222     fn suggest_change_mut(
 223         &self,
 224         obligation: &PredicateObligation<'tcx>,
 225         err: &mut Diagnostic,
 226         trait_pred: ty::PolyTraitPredicate<'tcx>,
 227     );
 228
 229     fn suggest_semicolon_removal(
 230         &self,
 231         obligation: &PredicateObligation<'tcx>,
 232         err: &mut Diagnostic,
 233         span: Span,
 234         trait_pred: ty::PolyTraitPredicate<'tcx>,
 235     ) -> bool;
 236
 237     fn return_type_span(&self, obligation: &PredicateObligation<'tcx>) -> Option<Span>;
 238
 239     fn suggest_impl_trait(
 240         &self,
 241         err: &mut Diagnostic,
 242         span: Span,
 243         obligation: &PredicateObligation<'tcx>,
 244         trait_pred: ty::PolyTraitPredicate<'tcx>,
 245     ) -> bool;
 246
 247     fn point_at_returns_when_relevant(
 248         &self,
 249         err: &mut Diagnostic,
 250         obligation: &PredicateObligation<'tcx>,
 251     );
 252
 253     fn report_closure_arg_mismatch(
 254         &self,
 255         span: Span,
 256         found_span: Option<Span>,
 257         found: ty::PolyTraitRef<'tcx>,
 258         expected: ty::PolyTraitRef<'tcx>,
 259         cause: &ObligationCauseCode<'tcx>,
 260     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
 261
 262     fn note_conflicting_closure_bounds(
 263         &self,
 264         cause: &ObligationCauseCode<'tcx>,
 265         err: &mut DiagnosticBuilder<'tcx, ErrorGuaranteed>,
 266     );
 267
 268     fn suggest_fully_qualified_path(
 269         &self,
 270         err: &mut Diagnostic,
 271         item_def_id: DefId,
 272         span: Span,
 273         trait_ref: DefId,
 274     );
 275
 276     fn maybe_note_obligation_cause_for_async_await(
 277         &self,
 278         err: &mut Diagnostic,
 279         obligation: &PredicateObligation<'tcx>,
 280     ) -> bool;
 281
 282     fn note_obligation_cause_for_async_await(
 283         &self,
 284         err: &mut Diagnostic,
 285         interior_or_upvar_span: GeneratorInteriorOrUpvar,
 286         is_async: bool,
 287         outer_generator: Option<DefId>,
 288         trait_pred: ty::TraitPredicate<'tcx>,
 289         target_ty: Ty<'tcx>,
 290         typeck_results: Option<&ty::TypeckResults<'tcx>>,
 291         obligation: &PredicateObligation<'tcx>,
 292         next_code: Option<&ObligationCauseCode<'tcx>>,
 293     );
 294
 295     fn note_obligation_cause_code<T>(
 296         &self,
 297         err: &mut Diagnostic,
 298         predicate: &T,
 299         param_env: ty::ParamEnv<'tcx>,
 300         cause_code: &ObligationCauseCode<'tcx>,
 301         obligated_types: &mut Vec<Ty<'tcx>>,
 302         seen_requirements: &mut FxHashSet<DefId>,
 303     ) where
 304         T: fmt::Display + ToPredicate<'tcx, T>;
 305
 306     /// Suggest to await before try: future? => future.await?
 307     fn suggest_await_before_try(
 308         &self,
 309         err: &mut Diagnostic,
 310         obligation: &PredicateObligation<'tcx>,
 311         trait_pred: ty::PolyTraitPredicate<'tcx>,
 312         span: Span,
 313     );
 314
 315     fn suggest_floating_point_literal(
 316         &self,
 317         obligation: &PredicateObligation<'tcx>,
 318         err: &mut Diagnostic,
 319         trait_ref: &ty::PolyTraitRef<'tcx>,
 320     );
 321
 322     fn suggest_derive(
 323         &self,
 324         obligation: &PredicateObligation<'tcx>,
 325         err: &mut Diagnostic,
 326         trait_pred: ty::PolyTraitPredicate<'tcx>,
 327     );
 328
 329     fn suggest_dereferencing_index(
 330         &self,
 331         obligation: &PredicateObligation<'tcx>,
 332         err: &mut Diagnostic,
 333         trait_pred: ty::PolyTraitPredicate<'tcx>,
 334     );
 335 }
 336
 337 fn predicate_constraint(generics: &hir::Generics<'_>, pred: ty::Predicate<'_>) -> (Span, String) {
 338     (
 339         generics.tail_span_for_predicate_suggestion(),
 340         format!("{} {}", generics.add_where_or_trailing_comma(), pred),
 341     )
 342 }
 343
 344 /// Type parameter needs more bounds. The trivial case is `T` `where T: Bound`, but
 345 /// it can also be an `impl Trait` param that needs to be decomposed to a type
 346 /// param for cleaner code.
 347 fn suggest_restriction<'tcx>(
 348     tcx: TyCtxt<'tcx>,
 349     hir_id: HirId,
 350     hir_generics: &hir::Generics<'tcx>,
 351     msg: &str,
 352     err: &mut Diagnostic,
 353     fn_sig: Option<&hir::FnSig<'_>>,
 354     projection: Option<&ty::ProjectionTy<'_>>,
 355     trait_pred: ty::PolyTraitPredicate<'tcx>,
 356     // When we are dealing with a trait, `super_traits` will be `Some`:
 357     // Given `trait T: A + B + C {}`
 358     //              -  ^^^^^^^^^ GenericBounds
 359     //              |
 360     //              &Ident
 361     super_traits: Option<(&Ident, &hir::GenericBounds<'_>)>,
 362 ) {
 363     if hir_generics.where_clause_span.from_expansion()
 364         || hir_generics.where_clause_span.desugaring_kind().is_some()
 365     {
 366         return;
 367     }
 368     let Some(item_id) = hir_id.as_owner() else { return; };
 369     let generics = tcx.generics_of(item_id);
 370     // Given `fn foo(t: impl Trait)` where `Trait` requires assoc type `A`...
 371     if let Some((param, bound_str, fn_sig)) =
 372         fn_sig.zip(projection).and_then(|(sig, p)| match p.self_ty().kind() {
 373             // Shenanigans to get the `Trait` from the `impl Trait`.
 374             ty::Param(param) => {
 375                 let param_def = generics.type_param(param, tcx);
 376                 if param_def.kind.is_synthetic() {
 377                     let bound_str =
 378                         param_def.name.as_str().strip_prefix("impl ")?.trim_start().to_string();
 379                     return Some((param_def, bound_str, sig));
 380                 }
 381                 None
 382             }
 383             _ => None,
 384         })
 385     {
 386         let type_param_name = hir_generics.params.next_type_param_name(Some(&bound_str));
 387         let trait_pred = trait_pred.fold_with(&mut ReplaceImplTraitFolder {
 388             tcx,
 389             param,
 390             replace_ty: ty::ParamTy::new(generics.count() as u32, Symbol::intern(&type_param_name))
 391                 .to_ty(tcx),
 392         });
 393         if !trait_pred.is_suggestable(tcx, false) {
 394             return;
 395         }
 396         // We know we have an `impl Trait` that doesn't satisfy a required projection.
 397
 398         // Find all of the occurrences of `impl Trait` for `Trait` in the function arguments'
 399         // types. There should be at least one, but there might be *more* than one. In that
 400         // case we could just ignore it and try to identify which one needs the restriction,
 401         // but instead we choose to suggest replacing all instances of `impl Trait` with `T`
 402         // where `T: Trait`.
 403         let mut ty_spans = vec![];
 404         for input in fn_sig.decl.inputs {
 405             ReplaceImplTraitVisitor { ty_spans: &mut ty_spans, param_did: param.def_id }
 406                 .visit_ty(input);
 407         }
 408         // The type param `T: Trait` we will suggest to introduce.
 409         let type_param = format!("{}: {}", type_param_name, bound_str);
 410
 411         let mut sugg = vec![
 412             if let Some(span) = hir_generics.span_for_param_suggestion() {
 413                 (span, format!(", {}", type_param))
 414             } else {
 415                 (hir_generics.span, format!("<{}>", type_param))
 416             },
 417             // `fn foo(t: impl Trait)`
 418             //                       ^ suggest `where <T as Trait>::A: Bound`
 419             predicate_constraint(hir_generics, trait_pred.to_predicate(tcx)),
 420         ];
 421         sugg.extend(ty_spans.into_iter().map(|s| (s, type_param_name.to_string())));
 422
 423         // Suggest `fn foo<T: Trait>(t: T) where <T as Trait>::A: Bound`.
 424         // FIXME: once `#![feature(associated_type_bounds)]` is stabilized, we should suggest
 425         // `fn foo(t: impl Trait<A: Bound>)` instead.
 426         err.multipart_suggestion(
 427             "introduce a type parameter with a trait bound instead of using `impl Trait`",
 428             sugg,
 429             Applicability::MaybeIncorrect,
 430         );
 431     } else {
 432         if !trait_pred.is_suggestable(tcx, false) {
 433             return;
 434         }
 435         // Trivial case: `T` needs an extra bound: `T: Bound`.
 436         let (sp, suggestion) = match (
 437             hir_generics
 438                 .params
 439                 .iter()
 440                 .find(|p| !matches!(p.kind, hir::GenericParamKind::Type { synthetic: true, .. })),
 441             super_traits,
 442         ) {
 443             (_, None) => predicate_constraint(hir_generics, trait_pred.to_predicate(tcx)),
 444             (None, Some((ident, []))) => (
 445                 ident.span.shrink_to_hi(),
 446                 format!(": {}", trait_pred.print_modifiers_and_trait_path()),
 447             ),
 448             (_, Some((_, [.., bounds]))) => (
 449                 bounds.span().shrink_to_hi(),
 450                 format!(" + {}", trait_pred.print_modifiers_and_trait_path()),
 451             ),
 452             (Some(_), Some((_, []))) => (
 453                 hir_generics.span.shrink_to_hi(),
 454                 format!(": {}", trait_pred.print_modifiers_and_trait_path()),
 455             ),
 456         };
 457
 458         err.span_suggestion_verbose(
 459             sp,
 460             &format!("consider further restricting {}", msg),
 461             suggestion,
 462             Applicability::MachineApplicable,
 463         );
 464     }
 465 }
 466
 467 impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 468     fn suggest_restricting_param_bound(
 469         &self,
 470         mut err: &mut Diagnostic,
 471         trait_pred: ty::PolyTraitPredicate<'tcx>,
 472         associated_ty: Option<(&'static str, Ty<'tcx>)>,
 473         body_id: hir::HirId,
 474     ) {
 475         let trait_pred = self.resolve_numeric_literals_with_default(trait_pred);
 476
 477         let self_ty = trait_pred.skip_binder().self_ty();
 478         let (param_ty, projection) = match self_ty.kind() {
 479             ty::Param(_) => (true, None),
 480             ty::Projection(projection) => (false, Some(projection)),
 481             _ => (false, None),
 482         };
 483
 484         // FIXME: Add check for trait bound that is already present, particularly `?Sized` so we
 485         //        don't suggest `T: Sized + ?Sized`.
 486         let mut hir_id = body_id;
 487         while let Some(node) = self.tcx.hir().find(hir_id) {
 488             match node {
 489                 hir::Node::Item(hir::Item {
 490                     ident,
 491                     kind: hir::ItemKind::Trait(_, _, generics, bounds, _),
 492                     ..
 493                 }) if self_ty == self.tcx.types.self_param => {
 494                     assert!(param_ty);
 495                     // Restricting `Self` for a single method.
 496                     suggest_restriction(
 497                         self.tcx,
 498                         hir_id,
 499                         &generics,
 500                         "`Self`",
 501                         err,
 502                         None,
 503                         projection,
 504                         trait_pred,
 505                         Some((ident, bounds)),
 506                     );
 507                     return;
 508                 }
 509
 510                 hir::Node::TraitItem(hir::TraitItem {
 511                     generics,
 512                     kind: hir::TraitItemKind::Fn(..),
 513                     ..
 514                 }) if self_ty == self.tcx.types.self_param => {
 515                     assert!(param_ty);
 516                     // Restricting `Self` for a single method.
 517                     suggest_restriction(
 518                         self.tcx, hir_id, &generics, "`Self`", err, None, projection, trait_pred,
 519                         None,
 520                     );
 521                     return;
 522                 }
 523
 524                 hir::Node::TraitItem(hir::TraitItem {
 525                     generics,
 526                     kind: hir::TraitItemKind::Fn(fn_sig, ..),
 527                     ..
 528                 })
 529                 | hir::Node::ImplItem(hir::ImplItem {
 530                     generics,
 531                     kind: hir::ImplItemKind::Fn(fn_sig, ..),
 532                     ..
 533                 })
 534                 | hir::Node::Item(hir::Item {
 535                     kind: hir::ItemKind::Fn(fn_sig, generics, _), ..
 536                 }) if projection.is_some() => {
 537                     // Missing restriction on associated type of type parameter (unmet projection).
 538                     suggest_restriction(
 539                         self.tcx,
 540                         hir_id,
 541                         &generics,
 542                         "the associated type",
 543                         err,
 544                         Some(fn_sig),
 545                         projection,
 546                         trait_pred,
 547                         None,
 548                     );
 549                     return;
 550                 }
 551                 hir::Node::Item(hir::Item {
 552                     kind:
 553                         hir::ItemKind::Trait(_, _, generics, ..)
 554                         | hir::ItemKind::Impl(hir::Impl { generics, .. }),
 555                     ..
 556                 }) if projection.is_some() => {
 557                     // Missing restriction on associated type of type parameter (unmet projection).
 558                     suggest_restriction(
 559                         self.tcx,
 560                         hir_id,
 561                         &generics,
 562                         "the associated type",
 563                         err,
 564                         None,
 565                         projection,
 566                         trait_pred,
 567                         None,
 568                     );
 569                     return;
 570                 }
 571
 572                 hir::Node::Item(hir::Item {
 573                     kind:
 574                         hir::ItemKind::Struct(_, generics)
 575                         | hir::ItemKind::Enum(_, generics)
 576                         | hir::ItemKind::Union(_, generics)
 577                         | hir::ItemKind::Trait(_, _, generics, ..)
 578                         | hir::ItemKind::Impl(hir::Impl { generics, .. })
 579                         | hir::ItemKind::Fn(_, generics, _)
 580                         | hir::ItemKind::TyAlias(_, generics)
 581                         | hir::ItemKind::TraitAlias(generics, _)
 582                         | hir::ItemKind::OpaqueTy(hir::OpaqueTy { generics, .. }),
 583                     ..
 584                 })
 585                 | hir::Node::TraitItem(hir::TraitItem { generics, .. })
 586                 | hir::Node::ImplItem(hir::ImplItem { generics, .. })
 587                     if param_ty =>
 588                 {
 589                     // We skip the 0'th subst (self) because we do not want
 590                     // to consider the predicate as not suggestible if the
 591                     // self type is an arg position `impl Trait` -- instead,
 592                     // we handle that by adding ` + Bound` below.
 593                     // FIXME(compiler-errors): It would be nice to do the same
 594                     // this that we do in `suggest_restriction` and pull the
 595                     // `impl Trait` into a new generic if it shows up somewhere
 596                     // else in the predicate.
 597                     if !trait_pred.skip_binder().trait_ref.substs[1..]
 598                         .iter()
 599                         .all(|g| g.is_suggestable(self.tcx, false))
 600                     {
 601                         return;
 602                     }
 603                     // Missing generic type parameter bound.
 604                     let param_name = self_ty.to_string();
 605                     let mut constraint = with_no_trimmed_paths!(
 606                         trait_pred.print_modifiers_and_trait_path().to_string()
 607                     );
 608
 609                     if let Some((name, term)) = associated_ty {
 610                         // FIXME: this case overlaps with code in TyCtxt::note_and_explain_type_err.
 611                         // That should be extracted into a helper function.
 612                         if constraint.ends_with('>') {
 613                             constraint = format!(
 614                                 "{}, {} = {}>",
 615                                 &constraint[..constraint.len() - 1],
 616                                 name,
 617                                 term
 618                             );
 619                         } else {
 620                             constraint.push_str(&format!("<{} = {}>", name, term));
 621                         }
 622                     }
 623
 624                     if suggest_constraining_type_param(
 625                         self.tcx,
 626                         generics,
 627                         &mut err,
 628                         &param_name,
 629                         &constraint,
 630                         Some(trait_pred.def_id()),
 631                     ) {
 632                         return;
 633                     }
 634                 }
 635
 636                 hir::Node::Item(hir::Item {
 637                     kind:
 638                         hir::ItemKind::Struct(_, generics)
 639                         | hir::ItemKind::Enum(_, generics)
 640                         | hir::ItemKind::Union(_, generics)
 641                         | hir::ItemKind::Trait(_, _, generics, ..)
 642                         | hir::ItemKind::Impl(hir::Impl { generics, .. })
 643                         | hir::ItemKind::Fn(_, generics, _)
 644                         | hir::ItemKind::TyAlias(_, generics)
 645                         | hir::ItemKind::TraitAlias(generics, _)
 646                         | hir::ItemKind::OpaqueTy(hir::OpaqueTy { generics, .. }),
 647                     ..
 648                 }) if !param_ty => {
 649                     // Missing generic type parameter bound.
 650                     if suggest_arbitrary_trait_bound(
 651                         self.tcx,
 652                         generics,
 653                         &mut err,
 654                         trait_pred,
 655                         associated_ty,
 656                     ) {
 657                         return;
 658                     }
 659                 }
 660                 hir::Node::Crate(..) => return,
 661
 662                 _ => {}
 663             }
 664
 665             hir_id = self.tcx.hir().get_parent_item(hir_id).into();
 666         }
 667     }
 668
 669     /// When after several dereferencing, the reference satisfies the trait
 670     /// binding. This function provides dereference suggestion for this
 671     /// specific situation.
 672     fn suggest_dereferences(
 673         &self,
 674         obligation: &PredicateObligation<'tcx>,
 675         err: &mut Diagnostic,
 676         trait_pred: ty::PolyTraitPredicate<'tcx>,
 677     ) -> bool {
 678         // It only make sense when suggesting dereferences for arguments
 679         let ObligationCauseCode::FunctionArgumentObligation { arg_hir_id, .. } = obligation.cause.code()
 680             else { return false; };
 681         let Some(typeck_results) = &self.typeck_results
 682             else { return false; };
 683         let hir::Node::Expr(expr) = self.tcx.hir().get(*arg_hir_id)
 684             else { return false; };
 685         let Some(arg_ty) = typeck_results.expr_ty_adjusted_opt(expr)
 686             else { return false; };
 687
 688         let span = obligation.cause.span;
 689         let mut real_trait_pred = trait_pred;
 690         let mut code = obligation.cause.code();
 691         while let Some((parent_code, parent_trait_pred)) = code.parent() {
 692             code = parent_code;
 693             if let Some(parent_trait_pred) = parent_trait_pred {
 694                 real_trait_pred = parent_trait_pred;
 695             }
 696
 697             let real_ty = real_trait_pred.self_ty();
 698             // We `erase_late_bound_regions` here because `make_subregion` does not handle
 699             // `ReLateBound`, and we don't particularly care about the regions.
 700             if self
 701                 .can_eq(obligation.param_env, self.tcx.erase_late_bound_regions(real_ty), arg_ty)
 702                 .is_err()
 703             {
 704                 continue;
 705             }
 706
 707             if let ty::Ref(region, base_ty, mutbl) = *real_ty.skip_binder().kind() {
 708                 let mut autoderef = Autoderef::new(
 709                     self,
 710                     obligation.param_env,
 711                     obligation.cause.body_id,
 712                     span,
 713                     base_ty,
 714                 );
 715                 if let Some(steps) = autoderef.find_map(|(ty, steps)| {
 716                     // Re-add the `&`
 717                     let ty = self.tcx.mk_ref(region, TypeAndMut { ty, mutbl });
 718
 719                     // Remapping bound vars here
 720                     let real_trait_pred_and_ty =
 721                         real_trait_pred.map_bound(|inner_trait_pred| (inner_trait_pred, ty));
 722                     let obligation = self.mk_trait_obligation_with_new_self_ty(
 723                         obligation.param_env,
 724                         real_trait_pred_and_ty,
 725                     );
 726                     Some(steps).filter(|_| self.predicate_may_hold(&obligation))
 727                 }) {
 728                     if steps > 0 {
 729                         // Don't care about `&mut` because `DerefMut` is used less
 730                         // often and user will not expect autoderef happens.
 731                         if let Some(hir::Node::Expr(hir::Expr {
 732                             kind:
 733                                 hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Not, expr),
 734                             ..
 735                         })) = self.tcx.hir().find(*arg_hir_id)
 736                         {
 737                             let derefs = "*".repeat(steps);
 738                             err.span_suggestion_verbose(
 739                                 expr.span.shrink_to_lo(),
 740                                 "consider dereferencing here",
 741                                 derefs,
 742                                 Applicability::MachineApplicable,
 743                             );
 744                             return true;
 745                         }
 746                     }
 747                 } else if real_trait_pred != trait_pred {
 748                     // This branch addresses #87437.
 749
 750                     // Remapping bound vars here
 751                     let real_trait_pred_and_base_ty =
 752                         real_trait_pred.map_bound(|inner_trait_pred| (inner_trait_pred, base_ty));
 753                     let obligation = self.mk_trait_obligation_with_new_self_ty(
 754                         obligation.param_env,
 755                         real_trait_pred_and_base_ty,
 756                     );
 757                     if self.predicate_may_hold(&obligation) {
 758                         err.span_suggestion_verbose(
 759                             span.shrink_to_lo(),
 760                             "consider dereferencing here",
 761                             "*",
 762                             Applicability::MachineApplicable,
 763                         );
 764                         return true;
 765                     }
 766                 }
 767             }
 768         }
 769         false
 770     }
 771
 772     /// Given a closure's `DefId`, return the given name of the closure.
 773     ///
 774     /// This doesn't account for reassignments, but it's only used for suggestions.
 775     fn get_closure_name(&self, def_id: DefId, err: &mut Diagnostic, msg: &str) -> Option<Symbol> {
 776         let get_name = |err: &mut Diagnostic, kind: &hir::PatKind<'_>| -> Option<Symbol> {
 777             // Get the local name of this closure. This can be inaccurate because
 778             // of the possibility of reassignment, but this should be good enough.
 779             match &kind {
 780                 hir::PatKind::Binding(hir::BindingAnnotation::NONE, _, ident, None) => {
 781                     Some(ident.name)
 782                 }
 783                 _ => {
 784                     err.note(msg);
 785                     None
 786                 }
 787             }
 788         };
 789
 790         let hir = self.tcx.hir();
 791         let hir_id = hir.local_def_id_to_hir_id(def_id.as_local()?);
 792         let parent_node = hir.get_parent_node(hir_id);
 793         match hir.find(parent_node) {
 794             Some(hir::Node::Stmt(hir::Stmt { kind: hir::StmtKind::Local(local), .. })) => {
 795                 get_name(err, &local.pat.kind)
 796             }
 797             // Different to previous arm because one is `&hir::Local` and the other
 798             // is `P<hir::Local>`.
 799             Some(hir::Node::Local(local)) => get_name(err, &local.pat.kind),
 800             _ => None,
 801         }
 802     }
 803
 804     /// We tried to apply the bound to an `fn` or closure. Check whether calling it would
 805     /// evaluate to a type that *would* satisfy the trait binding. If it would, suggest calling
 806     /// it: `bar(foo)` → `bar(foo())`. This case is *very* likely to be hit if `foo` is `async`.
 807     fn suggest_fn_call(
 808         &self,
 809         obligation: &PredicateObligation<'tcx>,
 810         err: &mut Diagnostic,
 811         trait_pred: ty::PolyTraitPredicate<'tcx>,
 812     ) -> bool {
 813         if let ty::PredicateKind::Trait(trait_pred) = obligation.predicate.kind().skip_binder()
 814             && Some(trait_pred.def_id()) == self.tcx.lang_items().sized_trait()
 815         {
 816             // Don't suggest calling to turn an unsized type into a sized type
 817             return false;
 818         }
 819
 820         // This is duplicated from `extract_callable_info` in typeck, which
 821         // relies on autoderef, so we can't use it here.
 822         let found = trait_pred.self_ty().skip_binder().peel_refs();
 823         let Some((def_id_or_name, output, inputs)) = (match *found.kind()
 824         {
 825             ty::FnPtr(fn_sig) => {
 826                 Some((DefIdOrName::Name("function pointer"), fn_sig.output(), fn_sig.inputs()))
 827             }
 828             ty::FnDef(def_id, _) => {
 829                 let fn_sig = found.fn_sig(self.tcx);
 830                 Some((DefIdOrName::DefId(def_id), fn_sig.output(), fn_sig.inputs()))
 831             }
 832             ty::Closure(def_id, substs) => {
 833                 let fn_sig = substs.as_closure().sig();
 834                 Some((
 835                     DefIdOrName::DefId(def_id),
 836                     fn_sig.output(),
 837                     fn_sig.inputs().map_bound(|inputs| &inputs[1..]),
 838                 ))
 839             }
 840             ty::Opaque(def_id, substs) => {
 841                 self.tcx.bound_item_bounds(def_id).subst(self.tcx, substs).iter().find_map(|pred| {
 842                     if let ty::PredicateKind::Projection(proj) = pred.kind().skip_binder()
 843                     && Some(proj.projection_ty.item_def_id) == self.tcx.lang_items().fn_once_output()
 844                     // args tuple will always be substs[1]
 845                     && let ty::Tuple(args) = proj.projection_ty.substs.type_at(1).kind()
 846                     {
 847                         Some((
 848                             DefIdOrName::DefId(def_id),
 849                             pred.kind().rebind(proj.term.ty().unwrap()),
 850                             pred.kind().rebind(args.as_slice()),
 851                         ))
 852                     } else {
 853                         None
 854                     }
 855                 })
 856             }
 857             ty::Dynamic(data, _, ty::Dyn) => {
 858                 data.iter().find_map(|pred| {
 859                     if let ty::ExistentialPredicate::Projection(proj) = pred.skip_binder()
 860                     && Some(proj.item_def_id) == self.tcx.lang_items().fn_once_output()
 861                     // for existential projection, substs are shifted over by 1
 862                     && let ty::Tuple(args) = proj.substs.type_at(0).kind()
 863                     {
 864                         Some((
 865                             DefIdOrName::Name("trait object"),
 866                             pred.rebind(proj.term.ty().unwrap()),
 867                             pred.rebind(args.as_slice()),
 868                         ))
 869                     } else {
 870                         None
 871                     }
 872                 })
 873             }
 874             ty::Param(_) => {
 875                 obligation.param_env.caller_bounds().iter().find_map(|pred| {
 876                     if let ty::PredicateKind::Projection(proj) = pred.kind().skip_binder()
 877                     && Some(proj.projection_ty.item_def_id) == self.tcx.lang_items().fn_once_output()
 878                     && proj.projection_ty.self_ty() == found
 879                     // args tuple will always be substs[1]
 880                     && let ty::Tuple(args) = proj.projection_ty.substs.type_at(1).kind()
 881                     {
 882                         Some((
 883                             DefIdOrName::Name("type parameter"),
 884                             pred.kind().rebind(proj.term.ty().unwrap()),
 885                             pred.kind().rebind(args.as_slice()),
 886                         ))
 887                     } else {
 888                         None
 889                     }
 890                 })
 891             }
 892             _ => None,
 893         }) else { return false; };
 894         let output = self.replace_bound_vars_with_fresh_vars(
 895             obligation.cause.span,
 896             LateBoundRegionConversionTime::FnCall,
 897             output,
 898         );
 899         let inputs = inputs.skip_binder().iter().map(|ty| {
 900             self.replace_bound_vars_with_fresh_vars(
 901                 obligation.cause.span,
 902                 LateBoundRegionConversionTime::FnCall,
 903                 inputs.rebind(*ty),
 904             )
 905         });
 906
 907         // Remapping bound vars here
 908         let trait_pred_and_self = trait_pred.map_bound(|trait_pred| (trait_pred, output));
 909
 910         let new_obligation =
 911             self.mk_trait_obligation_with_new_self_ty(obligation.param_env, trait_pred_and_self);
 912         if !self.predicate_must_hold_modulo_regions(&new_obligation) {
 913             return false;
 914         }
 915
 916         // Get the name of the callable and the arguments to be used in the suggestion.
 917         let hir = self.tcx.hir();
 918
 919         let msg = match def_id_or_name {
 920             DefIdOrName::DefId(def_id) => match self.tcx.def_kind(def_id) {
 921                 DefKind::Ctor(CtorOf::Struct, _) => {
 922                     "use parentheses to construct this tuple struct".to_string()
 923                 }
 924                 DefKind::Ctor(CtorOf::Variant, _) => {
 925                     "use parentheses to construct this tuple variant".to_string()
 926                 }
 927                 kind => format!("use parentheses to call this {}", kind.descr(def_id)),
 928             },
 929             DefIdOrName::Name(name) => format!("use parentheses to call this {name}"),
 930         };
 931
 932         let args = inputs
 933             .map(|ty| {
 934                 if ty.is_suggestable(self.tcx, false) {
 935                     format!("/* {ty} */")
 936                 } else {
 937                     "/* value */".to_string()
 938                 }
 939             })
 940             .collect::<Vec<_>>()
 941             .join(", ");
 942
 943         if matches!(obligation.cause.code(), ObligationCauseCode::FunctionArgumentObligation { .. })
 944             && obligation.cause.span.can_be_used_for_suggestions()
 945         {
 946             // When the obligation error has been ensured to have been caused by
 947             // an argument, the `obligation.cause.span` points at the expression
 948             // of the argument, so we can provide a suggestion. Otherwise, we give
 949             // a more general note.
 950             err.span_suggestion_verbose(
 951                 obligation.cause.span.shrink_to_hi(),
 952                 &msg,
 953                 format!("({args})"),
 954                 Applicability::HasPlaceholders,
 955             );
 956         } else if let DefIdOrName::DefId(def_id) = def_id_or_name {
 957             let name = match hir.get_if_local(def_id) {
 958                 Some(hir::Node::Expr(hir::Expr {
 959                     kind: hir::ExprKind::Closure(hir::Closure { fn_decl_span, .. }),
 960                     ..
 961                 })) => {
 962                     err.span_label(*fn_decl_span, "consider calling this closure");
 963                     let Some(name) = self.get_closure_name(def_id, err, &msg) else {
 964                         return false;
 965                     };
 966                     name.to_string()
 967                 }
 968                 Some(hir::Node::Item(hir::Item { ident, kind: hir::ItemKind::Fn(..), .. })) => {
 969                     err.span_label(ident.span, "consider calling this function");
 970                     ident.to_string()
 971                 }
 972                 Some(hir::Node::Ctor(..)) => {
 973                     let name = self.tcx.def_path_str(def_id);
 974                     err.span_label(
 975                         self.tcx.def_span(def_id),
 976                         format!("consider calling the constructor for `{}`", name),
 977                     );
 978                     name
 979                 }
 980                 _ => return false,
 981             };
 982             err.help(&format!("{msg}: `{name}({args})`"));
 983         }
 984         true
 985     }
 986
 987     fn suggest_add_reference_to_arg(
 988         &self,
 989         obligation: &PredicateObligation<'tcx>,
 990         err: &mut Diagnostic,
 991         poly_trait_pred: ty::PolyTraitPredicate<'tcx>,
 992         has_custom_message: bool,
 993     ) -> bool {
 994         let span = obligation.cause.span;
 995
 996         let code = if let ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } =
 997             obligation.cause.code()
 998         {
 999             &parent_code
1000         } else if let ObligationCauseCode::ItemObligation(_)
1001         | ObligationCauseCode::ExprItemObligation(..) = obligation.cause.code()
1002         {
1003             obligation.cause.code()
1004         } else if let ExpnKind::Desugaring(DesugaringKind::ForLoop) =
1005             span.ctxt().outer_expn_data().kind
1006         {
1007             obligation.cause.code()
1008         } else {
1009             return false;
1010         };
1011
1012         // List of traits for which it would be nonsensical to suggest borrowing.
1013         // For instance, immutable references are always Copy, so suggesting to
1014         // borrow would always succeed, but it's probably not what the user wanted.
1015         let mut never_suggest_borrow: Vec<_> =
1016             [LangItem::Copy, LangItem::Clone, LangItem::Unpin, LangItem::Sized]
1017                 .iter()
1018                 .filter_map(|lang_item| self.tcx.lang_items().get(*lang_item))
1019                 .collect();
1020
1021         if let Some(def_id) = self.tcx.get_diagnostic_item(sym::Send) {
1022             never_suggest_borrow.push(def_id);
1023         }
1024
1025         let param_env = obligation.param_env;
1026
1027         // Try to apply the original trait binding obligation by borrowing.
1028         let mut try_borrowing = |old_pred: ty::PolyTraitPredicate<'tcx>,
1029                                  blacklist: &[DefId]|
1030          -> bool {
1031             if blacklist.contains(&old_pred.def_id()) {
1032                 return false;
1033             }
1034             // We map bounds to `&T` and `&mut T`
1035             let trait_pred_and_imm_ref = old_pred.map_bound(|trait_pred| {
1036                 (
1037                     trait_pred,
1038                     self.tcx.mk_imm_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
1039                 )
1040             });
1041             let trait_pred_and_mut_ref = old_pred.map_bound(|trait_pred| {
1042                 (
1043                     trait_pred,
1044                     self.tcx.mk_mut_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
1045                 )
1046             });
1047
1048             let mk_result = |trait_pred_and_new_ty| {
1049                 let obligation =
1050                     self.mk_trait_obligation_with_new_self_ty(param_env, trait_pred_and_new_ty);
1051                 self.predicate_must_hold_modulo_regions(&obligation)
1052             };
1053             let imm_ref_self_ty_satisfies_pred = mk_result(trait_pred_and_imm_ref);
1054             let mut_ref_self_ty_satisfies_pred = mk_result(trait_pred_and_mut_ref);
1055
1056             let (ref_inner_ty_satisfies_pred, ref_inner_ty_mut) =
1057                 if let ObligationCauseCode::ItemObligation(_) | ObligationCauseCode::ExprItemObligation(..) = obligation.cause.code()
1058                     && let ty::Ref(_, ty, mutability) = old_pred.self_ty().skip_binder().kind()
1059                 {
1060                     (
1061                         mk_result(old_pred.map_bound(|trait_pred| (trait_pred, *ty))),
1062                         matches!(mutability, hir::Mutability::Mut),
1063                     )
1064                 } else {
1065                     (false, false)
1066                 };
1067
1068             if imm_ref_self_ty_satisfies_pred
1069                 || mut_ref_self_ty_satisfies_pred
1070                 || ref_inner_ty_satisfies_pred
1071             {
1072                 if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
1073                     // We don't want a borrowing suggestion on the fields in structs,
1074                     // ```
1075                     // struct Foo {
1076                     //  the_foos: Vec<Foo>
1077                     // }
1078                     // ```
1079                     if !matches!(
1080                         span.ctxt().outer_expn_data().kind,
1081                         ExpnKind::Root | ExpnKind::Desugaring(DesugaringKind::ForLoop)
1082                     ) {
1083                         return false;
1084                     }
1085                     if snippet.starts_with('&') {
1086                         // This is already a literal borrow and the obligation is failing
1087                         // somewhere else in the obligation chain. Do not suggest non-sense.
1088                         return false;
1089                     }
1090                     // We have a very specific type of error, where just borrowing this argument
1091                     // might solve the problem. In cases like this, the important part is the
1092                     // original type obligation, not the last one that failed, which is arbitrary.
1093                     // Because of this, we modify the error to refer to the original obligation and
1094                     // return early in the caller.
1095
1096                     let msg = format!("the trait bound `{}` is not satisfied", old_pred);
1097                     if has_custom_message {
1098                         err.note(&msg);
1099                     } else {
1100                         err.message =
1101                             vec![(rustc_errors::DiagnosticMessage::Str(msg), Style::NoStyle)];
1102                     }
1103                     err.span_label(
1104                         span,
1105                         format!(
1106                             "the trait `{}` is not implemented for `{}`",
1107                             old_pred.print_modifiers_and_trait_path(),
1108                             old_pred.self_ty().skip_binder(),
1109                         ),
1110                     );
1111
1112                     if imm_ref_self_ty_satisfies_pred && mut_ref_self_ty_satisfies_pred {
1113                         err.span_suggestions(
1114                             span.shrink_to_lo(),
1115                             "consider borrowing here",
1116                             ["&".to_string(), "&mut ".to_string()],
1117                             Applicability::MaybeIncorrect,
1118                         );
1119                     } else {
1120                         let is_mut = mut_ref_self_ty_satisfies_pred || ref_inner_ty_mut;
1121                         err.span_suggestion_verbose(
1122                             span.shrink_to_lo(),
1123                             &format!(
1124                                 "consider{} borrowing here",
1125                                 if is_mut { " mutably" } else { "" }
1126                             ),
1127                             format!("&{}", if is_mut { "mut " } else { "" }),
1128                             Applicability::MaybeIncorrect,
1129                         );
1130                     }
1131                     return true;
1132                 }
1133             }
1134             return false;
1135         };
1136
1137         if let ObligationCauseCode::ImplDerivedObligation(cause) = &*code {
1138             try_borrowing(cause.derived.parent_trait_pred, &[])
1139         } else if let ObligationCauseCode::BindingObligation(_, _)
1140         | ObligationCauseCode::ItemObligation(_)
1141         | ObligationCauseCode::ExprItemObligation(..)
1142         | ObligationCauseCode::ExprBindingObligation(..) = code
1143         {
1144             try_borrowing(poly_trait_pred, &never_suggest_borrow)
1145         } else {
1146             false
1147         }
1148     }
1149
1150     // Suggest borrowing the type
1151     fn suggest_borrowing_for_object_cast(
1152         &self,
1153         err: &mut Diagnostic,
1154         obligation: &PredicateObligation<'tcx>,
1155         self_ty: Ty<'tcx>,
1156         object_ty: Ty<'tcx>,
1157     ) {
1158         let ty::Dynamic(predicates, _, ty::Dyn) = object_ty.kind() else { return; };
1159         let self_ref_ty = self.tcx.mk_imm_ref(self.tcx.lifetimes.re_erased, self_ty);
1160
1161         for predicate in predicates.iter() {
1162             if !self.predicate_must_hold_modulo_regions(
1163                 &obligation.with(self.tcx, predicate.with_self_ty(self.tcx, self_ref_ty)),
1164             ) {
1165                 return;
1166             }
1167         }
1168
1169         err.span_suggestion(
1170             obligation.cause.span.shrink_to_lo(),
1171             &format!(
1172                 "consider borrowing the value, since `&{self_ty}` can be coerced into `{object_ty}`"
1173             ),
1174             "&",
1175             Applicability::MaybeIncorrect,
1176         );
1177     }
1178
1179     /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
1180     /// suggest removing these references until we reach a type that implements the trait.
1181     fn suggest_remove_reference(
1182         &self,
1183         obligation: &PredicateObligation<'tcx>,
1184         err: &mut Diagnostic,
1185         trait_pred: ty::PolyTraitPredicate<'tcx>,
1186     ) -> bool {
1187         let span = obligation.cause.span;
1188
1189         let mut suggested = false;
1190         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
1191             let refs_number =
1192                 snippet.chars().filter(|c| !c.is_whitespace()).take_while(|c| *c == '&').count();
1193             if let Some('\'') = snippet.chars().filter(|c| !c.is_whitespace()).nth(refs_number) {
1194                 // Do not suggest removal of borrow from type arguments.
1195                 return false;
1196             }
1197
1198             // Skipping binder here, remapping below
1199             let mut suggested_ty = trait_pred.self_ty().skip_binder();
1200
1201             for refs_remaining in 0..refs_number {
1202                 let ty::Ref(_, inner_ty, _) = suggested_ty.kind() else {
1203                     break;
1204                 };
1205                 suggested_ty = *inner_ty;
1206
1207                 // Remapping bound vars here
1208                 let trait_pred_and_suggested_ty =
1209                     trait_pred.map_bound(|trait_pred| (trait_pred, suggested_ty));
1210
1211                 let new_obligation = self.mk_trait_obligation_with_new_self_ty(
1212                     obligation.param_env,
1213                     trait_pred_and_suggested_ty,
1214                 );
1215
1216                 if self.predicate_may_hold(&new_obligation) {
1217                     let sp = self
1218                         .tcx
1219                         .sess
1220                         .source_map()
1221                         .span_take_while(span, |c| c.is_whitespace() || *c == '&');
1222
1223                     let remove_refs = refs_remaining + 1;
1224
1225                     let msg = if remove_refs == 1 {
1226                         "consider removing the leading `&`-reference".to_string()
1227                     } else {
1228                         format!("consider removing {} leading `&`-references", remove_refs)
1229                     };
1230
1231                     err.span_suggestion_short(sp, &msg, "", Applicability::MachineApplicable);
1232                     suggested = true;
1233                     break;
1234                 }
1235             }
1236         }
1237         suggested
1238     }
1239
1240     fn suggest_remove_await(&self, obligation: &PredicateObligation<'tcx>, err: &mut Diagnostic) {
1241         let span = obligation.cause.span;
1242
1243         if let ObligationCauseCode::AwaitableExpr(hir_id) = obligation.cause.code().peel_derives() {
1244             let hir = self.tcx.hir();
1245             if let Some(node) = hir_id.and_then(|hir_id| hir.find(hir_id)) {
1246                 if let hir::Node::Expr(expr) = node {
1247                     // FIXME: use `obligation.predicate.kind()...trait_ref.self_ty()` to see if we have `()`
1248                     // and if not maybe suggest doing something else? If we kept the expression around we
1249                     // could also check if it is an fn call (very likely) and suggest changing *that*, if
1250                     // it is from the local crate.
1251                     err.span_suggestion(
1252                         span,
1253                         "remove the `.await`",
1254                         "",
1255                         Applicability::MachineApplicable,
1256                     );
1257                     // FIXME: account for associated `async fn`s.
1258                     if let hir::Expr { span, kind: hir::ExprKind::Call(base, _), .. } = expr {
1259                         if let ty::PredicateKind::Trait(pred) =
1260                             obligation.predicate.kind().skip_binder()
1261                         {
1262                             err.span_label(
1263                                 *span,
1264                                 &format!("this call returns `{}`", pred.self_ty()),
1265                             );
1266                         }
1267                         if let Some(typeck_results) = &self.typeck_results
1268                             && let ty = typeck_results.expr_ty_adjusted(base)
1269                             && let ty::FnDef(def_id, _substs) = ty.kind()
1270                             && let Some(hir::Node::Item(hir::Item { ident, span, vis_span, .. })) =
1271                                 hir.get_if_local(*def_id)
1272                         {
1273                             let msg = format!(
1274                                 "alternatively, consider making `fn {}` asynchronous",
1275                                 ident
1276                             );
1277                             if vis_span.is_empty() {
1278                                 err.span_suggestion_verbose(
1279                                     span.shrink_to_lo(),
1280                                     &msg,
1281                                     "async ",
1282                                     Applicability::MaybeIncorrect,
1283                                 );
1284                             } else {
1285                                 err.span_suggestion_verbose(
1286                                     vis_span.shrink_to_hi(),
1287                                     &msg,
1288                                     " async",
1289                                     Applicability::MaybeIncorrect,
1290                                 );
1291                             }
1292                         }
1293                     }
1294                 }
1295             }
1296         }
1297     }
1298
1299     /// Check if the trait bound is implemented for a different mutability and note it in the
1300     /// final error.
1301     fn suggest_change_mut(
1302         &self,
1303         obligation: &PredicateObligation<'tcx>,
1304         err: &mut Diagnostic,
1305         trait_pred: ty::PolyTraitPredicate<'tcx>,
1306     ) {
1307         let points_at_arg = matches!(
1308             obligation.cause.code(),
1309             ObligationCauseCode::FunctionArgumentObligation { .. },
1310         );
1311
1312         let span = obligation.cause.span;
1313         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
1314             let refs_number =
1315                 snippet.chars().filter(|c| !c.is_whitespace()).take_while(|c| *c == '&').count();
1316             if let Some('\'') = snippet.chars().filter(|c| !c.is_whitespace()).nth(refs_number) {
1317                 // Do not suggest removal of borrow from type arguments.
1318                 return;
1319             }
1320             let trait_pred = self.resolve_vars_if_possible(trait_pred);
1321             if trait_pred.has_non_region_infer() {
1322                 // Do not ICE while trying to find if a reborrow would succeed on a trait with
1323                 // unresolved bindings.
1324                 return;
1325             }
1326
1327             // Skipping binder here, remapping below
1328             if let ty::Ref(region, t_type, mutability) = *trait_pred.skip_binder().self_ty().kind()
1329             {
1330                 let suggested_ty = match mutability {
1331                     hir::Mutability::Mut => self.tcx.mk_imm_ref(region, t_type),
1332                     hir::Mutability::Not => self.tcx.mk_mut_ref(region, t_type),
1333                 };
1334
1335                 // Remapping bound vars here
1336                 let trait_pred_and_suggested_ty =
1337                     trait_pred.map_bound(|trait_pred| (trait_pred, suggested_ty));
1338
1339                 let new_obligation = self.mk_trait_obligation_with_new_self_ty(
1340                     obligation.param_env,
1341                     trait_pred_and_suggested_ty,
1342                 );
1343                 let suggested_ty_would_satisfy_obligation = self
1344                     .evaluate_obligation_no_overflow(&new_obligation)
1345                     .must_apply_modulo_regions();
1346                 if suggested_ty_would_satisfy_obligation {
1347                     let sp = self
1348                         .tcx
1349                         .sess
1350                         .source_map()
1351                         .span_take_while(span, |c| c.is_whitespace() || *c == '&');
1352                     if points_at_arg && mutability == hir::Mutability::Not && refs_number > 0 {
1353                         err.span_suggestion_verbose(
1354                             sp,
1355                             "consider changing this borrow's mutability",
1356                             "&mut ",
1357                             Applicability::MachineApplicable,
1358                         );
1359                     } else {
1360                         err.note(&format!(
1361                             "`{}` is implemented for `{:?}`, but not for `{:?}`",
1362                             trait_pred.print_modifiers_and_trait_path(),
1363                             suggested_ty,
1364                             trait_pred.skip_binder().self_ty(),
1365                         ));
1366                     }
1367                 }
1368             }
1369         }
1370     }
1371
1372     fn suggest_semicolon_removal(
1373         &self,
1374         obligation: &PredicateObligation<'tcx>,
1375         err: &mut Diagnostic,
1376         span: Span,
1377         trait_pred: ty::PolyTraitPredicate<'tcx>,
1378     ) -> bool {
1379         let hir = self.tcx.hir();
1380         let parent_node = hir.get_parent_node(obligation.cause.body_id);
1381         let node = hir.find(parent_node);
1382         if let Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, _, body_id), .. })) = node
1383             && let hir::ExprKind::Block(blk, _) = &hir.body(*body_id).value.kind
1384             && sig.decl.output.span().overlaps(span)
1385             && blk.expr.is_none()
1386             && trait_pred.self_ty().skip_binder().is_unit()
1387             && let Some(stmt) = blk.stmts.last()
1388             && let hir::StmtKind::Semi(expr) = stmt.kind
1389             // Only suggest this if the expression behind the semicolon implements the predicate
1390             && let Some(typeck_results) = &self.typeck_results
1391             && let Some(ty) = typeck_results.expr_ty_opt(expr)
1392             && self.predicate_may_hold(&self.mk_trait_obligation_with_new_self_ty(
1393                 obligation.param_env, trait_pred.map_bound(|trait_pred| (trait_pred, ty))
1394             ))
1395         {
1396             err.span_label(
1397                 expr.span,
1398                 &format!(
1399                     "this expression has type `{}`, which implements `{}`",
1400                     ty,
1401                     trait_pred.print_modifiers_and_trait_path()
1402                 )
1403             );
1404             err.span_suggestion(
1405                 self.tcx.sess.source_map().end_point(stmt.span),
1406                 "remove this semicolon",
1407                 "",
1408                 Applicability::MachineApplicable
1409             );
1410             return true;
1411         }
1412         false
1413     }
1414
1415     fn return_type_span(&self, obligation: &PredicateObligation<'tcx>) -> Option<Span> {
1416         let hir = self.tcx.hir();
1417         let parent_node = hir.get_parent_node(obligation.cause.body_id);
1418         let Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, ..), .. })) = hir.find(parent_node) else {
1419             return None;
1420         };
1421
1422         if let hir::FnRetTy::Return(ret_ty) = sig.decl.output { Some(ret_ty.span) } else { None }
1423     }
1424
1425     /// If all conditions are met to identify a returned `dyn Trait`, suggest using `impl Trait` if
1426     /// applicable and signal that the error has been expanded appropriately and needs to be
1427     /// emitted.
1428     fn suggest_impl_trait(
1429         &self,
1430         err: &mut Diagnostic,
1431         span: Span,
1432         obligation: &PredicateObligation<'tcx>,
1433         trait_pred: ty::PolyTraitPredicate<'tcx>,
1434     ) -> bool {
1435         match obligation.cause.code().peel_derives() {
1436             // Only suggest `impl Trait` if the return type is unsized because it is `dyn Trait`.
1437             ObligationCauseCode::SizedReturnType => {}
1438             _ => return false,
1439         }
1440
1441         let hir = self.tcx.hir();
1442         let fn_hir_id = hir.get_parent_node(obligation.cause.body_id);
1443         let node = hir.find(fn_hir_id);
1444         let Some(hir::Node::Item(hir::Item {
1445             kind: hir::ItemKind::Fn(sig, _, body_id),
1446             ..
1447         })) = node
1448         else {
1449             return false;
1450         };
1451         let body = hir.body(*body_id);
1452         let trait_pred = self.resolve_vars_if_possible(trait_pred);
1453         let ty = trait_pred.skip_binder().self_ty();
1454         let is_object_safe = match ty.kind() {
1455             ty::Dynamic(predicates, _, ty::Dyn) => {
1456                 // If the `dyn Trait` is not object safe, do not suggest `Box<dyn Trait>`.
1457                 predicates
1458                     .principal_def_id()
1459                     .map_or(true, |def_id| self.tcx.object_safety_violations(def_id).is_empty())
1460             }
1461             // We only want to suggest `impl Trait` to `dyn Trait`s.
1462             // For example, `fn foo() -> str` needs to be filtered out.
1463             _ => return false,
1464         };
1465
1466         let hir::FnRetTy::Return(ret_ty) = sig.decl.output else {
1467             return false;
1468         };
1469
1470         // Use `TypeVisitor` instead of the output type directly to find the span of `ty` for
1471         // cases like `fn foo() -> (dyn Trait, i32) {}`.
1472         // Recursively look for `TraitObject` types and if there's only one, use that span to
1473         // suggest `impl Trait`.
1474
1475         // Visit to make sure there's a single `return` type to suggest `impl Trait`,
1476         // otherwise suggest using `Box<dyn Trait>` or an enum.
1477         let mut visitor = ReturnsVisitor::default();
1478         visitor.visit_body(&body);
1479
1480         let typeck_results = self.typeck_results.as_ref().unwrap();
1481         let Some(liberated_sig) = typeck_results.liberated_fn_sigs().get(fn_hir_id).copied() else { return false; };
1482
1483         let ret_types = visitor
1484             .returns
1485             .iter()
1486             .filter_map(|expr| Some((expr.span, typeck_results.node_type_opt(expr.hir_id)?)))
1487             .map(|(expr_span, ty)| (expr_span, self.resolve_vars_if_possible(ty)));
1488         let (last_ty, all_returns_have_same_type, only_never_return) = ret_types.clone().fold(
1489             (None, true, true),
1490             |(last_ty, mut same, only_never_return): (std::option::Option<Ty<'_>>, bool, bool),
1491              (_, ty)| {
1492                 let ty = self.resolve_vars_if_possible(ty);
1493                 same &=
1494                     !matches!(ty.kind(), ty::Error(_))
1495                         && last_ty.map_or(true, |last_ty| {
1496                             // FIXME: ideally we would use `can_coerce` here instead, but `typeck` comes
1497                             // *after* in the dependency graph.
1498                             match (ty.kind(), last_ty.kind()) {
1499                                 (Infer(InferTy::IntVar(_)), Infer(InferTy::IntVar(_)))
1500                                 | (Infer(InferTy::FloatVar(_)), Infer(InferTy::FloatVar(_)))
1501                                 | (Infer(InferTy::FreshIntTy(_)), Infer(InferTy::FreshIntTy(_)))
1502                                 | (
1503                                     Infer(InferTy::FreshFloatTy(_)),
1504                                     Infer(InferTy::FreshFloatTy(_)),
1505                                 ) => true,
1506                                 _ => ty == last_ty,
1507                             }
1508                         });
1509                 (Some(ty), same, only_never_return && matches!(ty.kind(), ty::Never))
1510             },
1511         );
1512         let mut spans_and_needs_box = vec![];
1513
1514         match liberated_sig.output().kind() {
1515             ty::Dynamic(predicates, _, ty::Dyn) => {
1516                 let cause = ObligationCause::misc(ret_ty.span, fn_hir_id);
1517                 let param_env = ty::ParamEnv::empty();
1518
1519                 if !only_never_return {
1520                     for (expr_span, return_ty) in ret_types {
1521                         let self_ty_satisfies_dyn_predicates = |self_ty| {
1522                             predicates.iter().all(|predicate| {
1523                                 let pred = predicate.with_self_ty(self.tcx, self_ty);
1524                                 let obl = Obligation::new(self.tcx, cause.clone(), param_env, pred);
1525                                 self.predicate_may_hold(&obl)
1526                             })
1527                         };
1528
1529                         if let ty::Adt(def, substs) = return_ty.kind()
1530                             && def.is_box()
1531                             && self_ty_satisfies_dyn_predicates(substs.type_at(0))
1532                         {
1533                             spans_and_needs_box.push((expr_span, false));
1534                         } else if self_ty_satisfies_dyn_predicates(return_ty) {
1535                             spans_and_needs_box.push((expr_span, true));
1536                         } else {
1537                             return false;
1538                         }
1539                     }
1540                 }
1541             }
1542             _ => return false,
1543         };
1544
1545         let sm = self.tcx.sess.source_map();
1546         if !ret_ty.span.overlaps(span) {
1547             return false;
1548         }
1549         let snippet = if let hir::TyKind::TraitObject(..) = ret_ty.kind {
1550             if let Ok(snippet) = sm.span_to_snippet(ret_ty.span) {
1551                 snippet
1552             } else {
1553                 return false;
1554             }
1555         } else {
1556             // Substitute the type, so we can print a fixup given `type Alias = dyn Trait`
1557             let name = liberated_sig.output().to_string();
1558             let name =
1559                 name.strip_prefix('(').and_then(|name| name.strip_suffix(')')).unwrap_or(&name);
1560             if !name.starts_with("dyn ") {
1561                 return false;
1562             }
1563             name.to_owned()
1564         };
1565
1566         err.code(error_code!(E0746));
1567         err.set_primary_message("return type cannot have an unboxed trait object");
1568         err.children.clear();
1569         let impl_trait_msg = "for information on `impl Trait`, see \
1570             <https://doc.rust-lang.org/book/ch10-02-traits.html\
1571             #returning-types-that-implement-traits>";
1572         let trait_obj_msg = "for information on trait objects, see \
1573             <https://doc.rust-lang.org/book/ch17-02-trait-objects.html\
1574             #using-trait-objects-that-allow-for-values-of-different-types>";
1575
1576         let has_dyn = snippet.split_whitespace().next().map_or(false, |s| s == "dyn");
1577         let trait_obj = if has_dyn { &snippet[4..] } else { &snippet };
1578         if only_never_return {
1579             // No return paths, probably using `panic!()` or similar.
1580             // Suggest `-> T`, `-> impl Trait`, and if `Trait` is object safe, `-> Box<dyn Trait>`.
1581             suggest_trait_object_return_type_alternatives(
1582                 err,
1583                 ret_ty.span,
1584                 trait_obj,
1585                 is_object_safe,
1586             );
1587         } else if let (Some(last_ty), true) = (last_ty, all_returns_have_same_type) {
1588             // Suggest `-> impl Trait`.
1589             err.span_suggestion(
1590                 ret_ty.span,
1591                 &format!(
1592                     "use `impl {1}` as the return type, as all return paths are of type `{}`, \
1593                      which implements `{1}`",
1594                     last_ty, trait_obj,
1595                 ),
1596                 format!("impl {}", trait_obj),
1597                 Applicability::MachineApplicable,
1598             );
1599             err.note(impl_trait_msg);
1600         } else {
1601             if is_object_safe {
1602                 // Suggest `-> Box<dyn Trait>` and `Box::new(returned_value)`.
1603                 err.multipart_suggestion(
1604                     "return a boxed trait object instead",
1605                     vec![
1606                         (ret_ty.span.shrink_to_lo(), "Box<".to_string()),
1607                         (span.shrink_to_hi(), ">".to_string()),
1608                     ],
1609                     Applicability::MaybeIncorrect,
1610                 );
1611                 for (span, needs_box) in spans_and_needs_box {
1612                     if needs_box {
1613                         err.multipart_suggestion(
1614                             "... and box this value",
1615                             vec![
1616                                 (span.shrink_to_lo(), "Box::new(".to_string()),
1617                                 (span.shrink_to_hi(), ")".to_string()),
1618                             ],
1619                             Applicability::MaybeIncorrect,
1620                         );
1621                     }
1622                 }
1623             } else {
1624                 // This is currently not possible to trigger because E0038 takes precedence, but
1625                 // leave it in for completeness in case anything changes in an earlier stage.
1626                 err.note(&format!(
1627                     "if trait `{}` were object-safe, you could return a trait object",
1628                     trait_obj,
1629                 ));
1630             }
1631             err.note(trait_obj_msg);
1632             err.note(&format!(
1633                 "if all the returned values were of the same type you could use `impl {}` as the \
1634                  return type",
1635                 trait_obj,
1636             ));
1637             err.note(impl_trait_msg);
1638             err.note("you can create a new `enum` with a variant for each returned type");
1639         }
1640         true
1641     }
1642
1643     fn point_at_returns_when_relevant(
1644         &self,
1645         err: &mut Diagnostic,
1646         obligation: &PredicateObligation<'tcx>,
1647     ) {
1648         match obligation.cause.code().peel_derives() {
1649             ObligationCauseCode::SizedReturnType => {}
1650             _ => return,
1651         }
1652
1653         let hir = self.tcx.hir();
1654         let parent_node = hir.get_parent_node(obligation.cause.body_id);
1655         let node = hir.find(parent_node);
1656         if let Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(_, _, body_id), .. })) =
1657             node
1658         {
1659             let body = hir.body(*body_id);
1660             // Point at all the `return`s in the function as they have failed trait bounds.
1661             let mut visitor = ReturnsVisitor::default();
1662             visitor.visit_body(&body);
1663             let typeck_results = self.typeck_results.as_ref().unwrap();
1664             for expr in &visitor.returns {
1665                 if let Some(returned_ty) = typeck_results.node_type_opt(expr.hir_id) {
1666                     let ty = self.resolve_vars_if_possible(returned_ty);
1667                     err.span_label(expr.span, &format!("this returned value is of type `{}`", ty));
1668                 }
1669             }
1670         }
1671     }
1672
1673     fn report_closure_arg_mismatch(
1674         &self,
1675         span: Span,
1676         found_span: Option<Span>,
1677         found: ty::PolyTraitRef<'tcx>,
1678         expected: ty::PolyTraitRef<'tcx>,
1679         cause: &ObligationCauseCode<'tcx>,
1680     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
1681         pub(crate) fn build_fn_sig_ty<'tcx>(
1682             infcx: &InferCtxt<'tcx>,
1683             trait_ref: ty::PolyTraitRef<'tcx>,
1684         ) -> Ty<'tcx> {
1685             let inputs = trait_ref.skip_binder().substs.type_at(1);
1686             let sig = match inputs.kind() {
1687                 ty::Tuple(inputs)
1688                     if infcx.tcx.fn_trait_kind_from_lang_item(trait_ref.def_id()).is_some() =>
1689                 {
1690                     infcx.tcx.mk_fn_sig(
1691                         inputs.iter(),
1692                         infcx.next_ty_var(TypeVariableOrigin {
1693                             span: DUMMY_SP,
1694                             kind: TypeVariableOriginKind::MiscVariable,
1695                         }),
1696                         false,
1697                         hir::Unsafety::Normal,
1698                         abi::Abi::Rust,
1699                     )
1700                 }
1701                 _ => infcx.tcx.mk_fn_sig(
1702                     std::iter::once(inputs),
1703                     infcx.next_ty_var(TypeVariableOrigin {
1704                         span: DUMMY_SP,
1705                         kind: TypeVariableOriginKind::MiscVariable,
1706                     }),
1707                     false,
1708                     hir::Unsafety::Normal,
1709                     abi::Abi::Rust,
1710                 ),
1711             };
1712
1713             infcx.tcx.mk_fn_ptr(trait_ref.rebind(sig))
1714         }
1715
1716         let argument_kind = match expected.skip_binder().self_ty().kind() {
1717             ty::Closure(..) => "closure",
1718             ty::Generator(..) => "generator",
1719             _ => "function",
1720         };
1721         let mut err = struct_span_err!(
1722             self.tcx.sess,
1723             span,
1724             E0631,
1725             "type mismatch in {argument_kind} arguments",
1726         );
1727
1728         err.span_label(span, "expected due to this");
1729
1730         let found_span = found_span.unwrap_or(span);
1731         err.span_label(found_span, "found signature defined here");
1732
1733         let expected = build_fn_sig_ty(self, expected);
1734         let found = build_fn_sig_ty(self, found);
1735
1736         let (expected_str, found_str) = self.cmp(expected, found);
1737
1738         let signature_kind = format!("{argument_kind} signature");
1739         err.note_expected_found(&signature_kind, expected_str, &signature_kind, found_str);
1740
1741         self.note_conflicting_closure_bounds(cause, &mut err);
1742
1743         err
1744     }
1745
1746     // Add a note if there are two `Fn`-family bounds that have conflicting argument
1747     // requirements, which will always cause a closure to have a type error.
1748     fn note_conflicting_closure_bounds(
1749         &self,
1750         cause: &ObligationCauseCode<'tcx>,
1751         err: &mut DiagnosticBuilder<'tcx, ErrorGuaranteed>,
1752     ) {
1753         // First, look for an `ExprBindingObligation`, which means we can get
1754         // the unsubstituted predicate list of the called function. And check
1755         // that the predicate that we failed to satisfy is a `Fn`-like trait.
1756         if let ObligationCauseCode::ExprBindingObligation(def_id, _, _, idx) = cause
1757             && let predicates = self.tcx.predicates_of(def_id).instantiate_identity(self.tcx)
1758             && let Some(pred) = predicates.predicates.get(*idx)
1759             && let ty::PredicateKind::Trait(trait_pred) = pred.kind().skip_binder()
1760             && ty::ClosureKind::from_def_id(self.tcx, trait_pred.def_id()).is_some()
1761         {
1762             let expected_self =
1763                 self.tcx.anonymize_late_bound_regions(pred.kind().rebind(trait_pred.self_ty()));
1764             let expected_substs = self
1765                 .tcx
1766                 .anonymize_late_bound_regions(pred.kind().rebind(trait_pred.trait_ref.substs));
1767
1768             // Find another predicate whose self-type is equal to the expected self type,
1769             // but whose substs don't match.
1770             let other_pred = std::iter::zip(&predicates.predicates, &predicates.spans)
1771                 .enumerate()
1772                 .find(|(other_idx, (pred, _))| match pred.kind().skip_binder() {
1773                     ty::PredicateKind::Trait(trait_pred)
1774                         if ty::ClosureKind::from_def_id(self.tcx, trait_pred.def_id())
1775                             .is_some()
1776                             && other_idx != idx
1777                             // Make sure that the self type matches
1778                             // (i.e. constraining this closure)
1779                             && expected_self
1780                                 == self.tcx.anonymize_late_bound_regions(
1781                                     pred.kind().rebind(trait_pred.self_ty()),
1782                                 )
1783                             // But the substs don't match (i.e. incompatible args)
1784                             && expected_substs
1785                                 != self.tcx.anonymize_late_bound_regions(
1786                                     pred.kind().rebind(trait_pred.trait_ref.substs),
1787                                 ) =>
1788                     {
1789                         true
1790                     }
1791                     _ => false,
1792                 });
1793             // If we found one, then it's very likely the cause of the error.
1794             if let Some((_, (_, other_pred_span))) = other_pred {
1795                 err.span_note(
1796                     *other_pred_span,
1797                     "closure inferred to have a different signature due to this bound",
1798                 );
1799             }
1800         }
1801     }
1802
1803     fn suggest_fully_qualified_path(
1804         &self,
1805         err: &mut Diagnostic,
1806         item_def_id: DefId,
1807         span: Span,
1808         trait_ref: DefId,
1809     ) {
1810         if let Some(assoc_item) = self.tcx.opt_associated_item(item_def_id) {
1811             if let ty::AssocKind::Const | ty::AssocKind::Type = assoc_item.kind {
1812                 err.note(&format!(
1813                     "{}s cannot be accessed directly on a `trait`, they can only be \
1814                         accessed through a specific `impl`",
1815                     assoc_item.kind.as_def_kind().descr(item_def_id)
1816                 ));
1817                 err.span_suggestion(
1818                     span,
1819                     "use the fully qualified path to an implementation",
1820                     format!("<Type as {}>::{}", self.tcx.def_path_str(trait_ref), assoc_item.name),
1821                     Applicability::HasPlaceholders,
1822                 );
1823             }
1824         }
1825     }
1826
1827     /// Adds an async-await specific note to the diagnostic when the future does not implement
1828     /// an auto trait because of a captured type.
1829     ///
1830     /// ```text
1831     /// note: future does not implement `Qux` as this value is used across an await
1832     ///   --> $DIR/issue-64130-3-other.rs:17:5
1833     ///    |
1834     /// LL |     let x = Foo;
1835     ///    |         - has type `Foo`
1836     /// LL |     baz().await;
1837     ///    |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
1838     /// LL | }
1839     ///    | - `x` is later dropped here
1840     /// ```
1841     ///
1842     /// When the diagnostic does not implement `Send` or `Sync` specifically, then the diagnostic
1843     /// is "replaced" with a different message and a more specific error.
1844     ///
1845     /// ```text
1846     /// error: future cannot be sent between threads safely
1847     ///   --> $DIR/issue-64130-2-send.rs:21:5
1848     ///    |
1849     /// LL | fn is_send<T: Send>(t: T) { }
1850     ///    |               ---- required by this bound in `is_send`
1851     /// ...
1852     /// LL |     is_send(bar());
1853     ///    |     ^^^^^^^ future returned by `bar` is not send
1854     ///    |
1855     ///    = help: within `impl std::future::Future`, the trait `std::marker::Send` is not
1856     ///            implemented for `Foo`
1857     /// note: future is not send as this value is used across an await
1858     ///   --> $DIR/issue-64130-2-send.rs:15:5
1859     ///    |
1860     /// LL |     let x = Foo;
1861     ///    |         - has type `Foo`
1862     /// LL |     baz().await;
1863     ///    |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
1864     /// LL | }
1865     ///    | - `x` is later dropped here
1866     /// ```
1867     ///
1868     /// Returns `true` if an async-await specific note was added to the diagnostic.
1869     #[instrument(level = "debug", skip_all, fields(?obligation.predicate, ?obligation.cause.span))]
1870     fn maybe_note_obligation_cause_for_async_await(
1871         &self,
1872         err: &mut Diagnostic,
1873         obligation: &PredicateObligation<'tcx>,
1874     ) -> bool {
1875         let hir = self.tcx.hir();
1876
1877         // Attempt to detect an async-await error by looking at the obligation causes, looking
1878         // for a generator to be present.
1879         //
1880         // When a future does not implement a trait because of a captured type in one of the
1881         // generators somewhere in the call stack, then the result is a chain of obligations.
1882         //
1883         // Given an `async fn` A that calls an `async fn` B which captures a non-send type and that
1884         // future is passed as an argument to a function C which requires a `Send` type, then the
1885         // chain looks something like this:
1886         //
1887         // - `BuiltinDerivedObligation` with a generator witness (B)
1888         // - `BuiltinDerivedObligation` with a generator (B)
1889         // - `BuiltinDerivedObligation` with `std::future::GenFuture` (B)
1890         // - `BuiltinDerivedObligation` with `impl std::future::Future` (B)
1891         // - `BuiltinDerivedObligation` with `impl std::future::Future` (B)
1892         // - `BuiltinDerivedObligation` with a generator witness (A)
1893         // - `BuiltinDerivedObligation` with a generator (A)
1894         // - `BuiltinDerivedObligation` with `std::future::GenFuture` (A)
1895         // - `BuiltinDerivedObligation` with `impl std::future::Future` (A)
1896         // - `BuiltinDerivedObligation` with `impl std::future::Future` (A)
1897         // - `BindingObligation` with `impl_send (Send requirement)
1898         //
1899         // The first obligation in the chain is the most useful and has the generator that captured
1900         // the type. The last generator (`outer_generator` below) has information about where the
1901         // bound was introduced. At least one generator should be present for this diagnostic to be
1902         // modified.
1903         let (mut trait_ref, mut target_ty) = match obligation.predicate.kind().skip_binder() {
1904             ty::PredicateKind::Trait(p) => (Some(p), Some(p.self_ty())),
1905             _ => (None, None),
1906         };
1907         let mut generator = None;
1908         let mut outer_generator = None;
1909         let mut next_code = Some(obligation.cause.code());
1910
1911         let mut seen_upvar_tys_infer_tuple = false;
1912
1913         while let Some(code) = next_code {
1914             debug!(?code);
1915             match code {
1916                 ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
1917                     next_code = Some(parent_code);
1918                 }
1919                 ObligationCauseCode::ImplDerivedObligation(cause) => {
1920                     let ty = cause.derived.parent_trait_pred.skip_binder().self_ty();
1921                     debug!(
1922                         parent_trait_ref = ?cause.derived.parent_trait_pred,
1923                         self_ty.kind = ?ty.kind(),
1924                         "ImplDerived",
1925                     );
1926
1927                     match *ty.kind() {
1928                         ty::Generator(did, ..) => {
1929                             generator = generator.or(Some(did));
1930                             outer_generator = Some(did);
1931                         }
1932                         ty::GeneratorWitness(..) => {}
1933                         ty::Tuple(_) if !seen_upvar_tys_infer_tuple => {
1934                             // By introducing a tuple of upvar types into the chain of obligations
1935                             // of a generator, the first non-generator item is now the tuple itself,
1936                             // we shall ignore this.
1937
1938                             seen_upvar_tys_infer_tuple = true;
1939                         }
1940                         _ if generator.is_none() => {
1941                             trait_ref = Some(cause.derived.parent_trait_pred.skip_binder());
1942                             target_ty = Some(ty);
1943                         }
1944                         _ => {}
1945                     }
1946
1947                     next_code = Some(&cause.derived.parent_code);
1948                 }
1949                 ObligationCauseCode::DerivedObligation(derived_obligation)
1950                 | ObligationCauseCode::BuiltinDerivedObligation(derived_obligation) => {
1951                     let ty = derived_obligation.parent_trait_pred.skip_binder().self_ty();
1952                     debug!(
1953                         parent_trait_ref = ?derived_obligation.parent_trait_pred,
1954                         self_ty.kind = ?ty.kind(),
1955                     );
1956
1957                     match *ty.kind() {
1958                         ty::Generator(did, ..) => {
1959                             generator = generator.or(Some(did));
1960                             outer_generator = Some(did);
1961                         }
1962                         ty::GeneratorWitness(..) => {}
1963                         ty::Tuple(_) if !seen_upvar_tys_infer_tuple => {
1964                             // By introducing a tuple of upvar types into the chain of obligations
1965                             // of a generator, the first non-generator item is now the tuple itself,
1966                             // we shall ignore this.
1967
1968                             seen_upvar_tys_infer_tuple = true;
1969                         }
1970                         _ if generator.is_none() => {
1971                             trait_ref = Some(derived_obligation.parent_trait_pred.skip_binder());
1972                             target_ty = Some(ty);
1973                         }
1974                         _ => {}
1975                     }
1976
1977                     next_code = Some(&derived_obligation.parent_code);
1978                 }
1979                 _ => break,
1980             }
1981         }
1982
1983         // Only continue if a generator was found.
1984         debug!(?generator, ?trait_ref, ?target_ty);
1985         let (Some(generator_did), Some(trait_ref), Some(target_ty)) = (generator, trait_ref, target_ty) else {
1986             return false;
1987         };
1988
1989         let span = self.tcx.def_span(generator_did);
1990
1991         let generator_did_root = self.tcx.typeck_root_def_id(generator_did);
1992         debug!(
1993             ?generator_did,
1994             ?generator_did_root,
1995             typeck_results.hir_owner = ?self.typeck_results.as_ref().map(|t| t.hir_owner),
1996             ?span,
1997         );
1998
1999         let generator_body = generator_did
2000             .as_local()
2001             .and_then(|def_id| hir.maybe_body_owned_by(def_id))
2002             .map(|body_id| hir.body(body_id));
2003         let is_async = self
2004             .tcx
2005             .generator_kind(generator_did)
2006             .map(|generator_kind| matches!(generator_kind, hir::GeneratorKind::Async(..)))
2007             .unwrap_or(false);
2008         let mut visitor = AwaitsVisitor::default();
2009         if let Some(body) = generator_body {
2010             visitor.visit_body(body);
2011         }
2012         debug!(awaits = ?visitor.awaits);
2013
2014         // Look for a type inside the generator interior that matches the target type to get
2015         // a span.
2016         let target_ty_erased = self.tcx.erase_regions(target_ty);
2017         let ty_matches = |ty| -> bool {
2018             // Careful: the regions for types that appear in the
2019             // generator interior are not generally known, so we
2020             // want to erase them when comparing (and anyway,
2021             // `Send` and other bounds are generally unaffected by
2022             // the choice of region).  When erasing regions, we
2023             // also have to erase late-bound regions. This is
2024             // because the types that appear in the generator
2025             // interior generally contain "bound regions" to
2026             // represent regions that are part of the suspended
2027             // generator frame. Bound regions are preserved by
2028             // `erase_regions` and so we must also call
2029             // `erase_late_bound_regions`.
2030             let ty_erased = self.tcx.erase_late_bound_regions(ty);
2031             let ty_erased = self.tcx.erase_regions(ty_erased);
2032             let eq = ty_erased == target_ty_erased;
2033             debug!(?ty_erased, ?target_ty_erased, ?eq);
2034             eq
2035         };
2036
2037         // Get the typeck results from the infcx if the generator is the function we are currently
2038         // type-checking; otherwise, get them by performing a query.  This is needed to avoid
2039         // cycles. If we can't use resolved types because the generator comes from another crate,
2040         // we still provide a targeted error but without all the relevant spans.
2041         let generator_data = match &self.typeck_results {
2042             Some(t) if t.hir_owner.to_def_id() == generator_did_root => GeneratorData::Local(&t),
2043             _ if generator_did.is_local() => {
2044                 GeneratorData::Local(self.tcx.typeck(generator_did.expect_local()))
2045             }
2046             _ if let Some(generator_diag_data) = self.tcx.generator_diagnostic_data(generator_did) => {
2047                 GeneratorData::Foreign(generator_diag_data)
2048             }
2049             _ => return false,
2050         };
2051
2052         let mut interior_or_upvar_span = None;
2053
2054         let from_awaited_ty = generator_data.get_from_await_ty(visitor, hir, ty_matches);
2055         debug!(?from_awaited_ty);
2056
2057         // The generator interior types share the same binders
2058         if let Some(cause) =
2059             generator_data.get_generator_interior_types().skip_binder().iter().find(
2060                 |ty::GeneratorInteriorTypeCause { ty, .. }| {
2061                     ty_matches(generator_data.get_generator_interior_types().rebind(*ty))
2062                 },
2063             )
2064         {
2065             let ty::GeneratorInteriorTypeCause { span, scope_span, yield_span, expr, .. } = cause;
2066
2067             interior_or_upvar_span = Some(GeneratorInteriorOrUpvar::Interior(
2068                 *span,
2069                 Some((*scope_span, *yield_span, *expr, from_awaited_ty)),
2070             ));
2071         }
2072
2073         if interior_or_upvar_span.is_none() {
2074             interior_or_upvar_span =
2075                 generator_data.try_get_upvar_span(&self, generator_did, ty_matches);
2076         }
2077
2078         if interior_or_upvar_span.is_none() && generator_data.is_foreign() {
2079             interior_or_upvar_span = Some(GeneratorInteriorOrUpvar::Interior(span, None));
2080         }
2081
2082         debug!(?interior_or_upvar_span);
2083         if let Some(interior_or_upvar_span) = interior_or_upvar_span {
2084             let typeck_results = match generator_data {
2085                 GeneratorData::Local(typeck_results) => Some(typeck_results),
2086                 GeneratorData::Foreign(_) => None,
2087             };
2088             self.note_obligation_cause_for_async_await(
2089                 err,
2090                 interior_or_upvar_span,
2091                 is_async,
2092                 outer_generator,
2093                 trait_ref,
2094                 target_ty,
2095                 typeck_results,
2096                 obligation,
2097                 next_code,
2098             );
2099             true
2100         } else {
2101             false
2102         }
2103     }
2104
2105     /// Unconditionally adds the diagnostic note described in
2106     /// `maybe_note_obligation_cause_for_async_await`'s documentation comment.
2107     #[instrument(level = "debug", skip_all)]
2108     fn note_obligation_cause_for_async_await(
2109         &self,
2110         err: &mut Diagnostic,
2111         interior_or_upvar_span: GeneratorInteriorOrUpvar,
2112         is_async: bool,
2113         outer_generator: Option<DefId>,
2114         trait_pred: ty::TraitPredicate<'tcx>,
2115         target_ty: Ty<'tcx>,
2116         typeck_results: Option<&ty::TypeckResults<'tcx>>,
2117         obligation: &PredicateObligation<'tcx>,
2118         next_code: Option<&ObligationCauseCode<'tcx>>,
2119     ) {
2120         let source_map = self.tcx.sess.source_map();
2121
2122         let (await_or_yield, an_await_or_yield) =
2123             if is_async { ("await", "an await") } else { ("yield", "a yield") };
2124         let future_or_generator = if is_async { "future" } else { "generator" };
2125
2126         // Special case the primary error message when send or sync is the trait that was
2127         // not implemented.
2128         let hir = self.tcx.hir();
2129         let trait_explanation = if let Some(name @ (sym::Send | sym::Sync)) =
2130             self.tcx.get_diagnostic_name(trait_pred.def_id())
2131         {
2132             let (trait_name, trait_verb) =
2133                 if name == sym::Send { ("`Send`", "sent") } else { ("`Sync`", "shared") };
2134
2135             err.clear_code();
2136             err.set_primary_message(format!(
2137                 "{} cannot be {} between threads safely",
2138                 future_or_generator, trait_verb
2139             ));
2140
2141             let original_span = err.span.primary_span().unwrap();
2142             let mut span = MultiSpan::from_span(original_span);
2143
2144             let message = outer_generator
2145                 .and_then(|generator_did| {
2146                     Some(match self.tcx.generator_kind(generator_did).unwrap() {
2147                         GeneratorKind::Gen => format!("generator is not {}", trait_name),
2148                         GeneratorKind::Async(AsyncGeneratorKind::Fn) => self
2149                             .tcx
2150                             .parent(generator_did)
2151                             .as_local()
2152                             .map(|parent_did| hir.local_def_id_to_hir_id(parent_did))
2153                             .and_then(|parent_hir_id| hir.opt_name(parent_hir_id))
2154                             .map(|name| {
2155                                 format!("future returned by `{}` is not {}", name, trait_name)
2156                             })?,
2157                         GeneratorKind::Async(AsyncGeneratorKind::Block) => {
2158                             format!("future created by async block is not {}", trait_name)
2159                         }
2160                         GeneratorKind::Async(AsyncGeneratorKind::Closure) => {
2161                             format!("future created by async closure is not {}", trait_name)
2162                         }
2163                     })
2164                 })
2165                 .unwrap_or_else(|| format!("{} is not {}", future_or_generator, trait_name));
2166
2167             span.push_span_label(original_span, message);
2168             err.set_span(span);
2169
2170             format!("is not {}", trait_name)
2171         } else {
2172             format!("does not implement `{}`", trait_pred.print_modifiers_and_trait_path())
2173         };
2174
2175         let mut explain_yield = |interior_span: Span,
2176                                  yield_span: Span,
2177                                  scope_span: Option<Span>| {
2178             let mut span = MultiSpan::from_span(yield_span);
2179             if let Ok(snippet) = source_map.span_to_snippet(interior_span) {
2180                 // #70935: If snippet contains newlines, display "the value" instead
2181                 // so that we do not emit complex diagnostics.
2182                 let snippet = &format!("`{}`", snippet);
2183                 let snippet = if snippet.contains('\n') { "the value" } else { snippet };
2184                 // note: future is not `Send` as this value is used across an await
2185                 //   --> $DIR/issue-70935-complex-spans.rs:13:9
2186                 //    |
2187                 // LL |            baz(|| async {
2188                 //    |  ______________-
2189                 //    | |
2190                 //    | |
2191                 // LL | |              foo(tx.clone());
2192                 // LL | |          }).await;
2193                 //    | |          - ^^^^^^ await occurs here, with value maybe used later
2194                 //    | |__________|
2195                 //    |            has type `closure` which is not `Send`
2196                 // note: value is later dropped here
2197                 // LL | |          }).await;
2198                 //    | |                  ^
2199                 //
2200                 span.push_span_label(
2201                     yield_span,
2202                     format!("{} occurs here, with {} maybe used later", await_or_yield, snippet),
2203                 );
2204                 span.push_span_label(
2205                     interior_span,
2206                     format!("has type `{}` which {}", target_ty, trait_explanation),
2207                 );
2208                 // If available, use the scope span to annotate the drop location.
2209                 let mut scope_note = None;
2210                 if let Some(scope_span) = scope_span {
2211                     let scope_span = source_map.end_point(scope_span);
2212
2213                     let msg = format!("{} is later dropped here", snippet);
2214                     if source_map.is_multiline(yield_span.between(scope_span)) {
2215                         span.push_span_label(scope_span, msg);
2216                     } else {
2217                         scope_note = Some((scope_span, msg));
2218                     }
2219                 }
2220                 err.span_note(
2221                     span,
2222                     &format!(
2223                         "{} {} as this value is used across {}",
2224                         future_or_generator, trait_explanation, an_await_or_yield
2225                     ),
2226                 );
2227                 if let Some((span, msg)) = scope_note {
2228                     err.span_note(span, &msg);
2229                 }
2230             }
2231         };
2232         match interior_or_upvar_span {
2233             GeneratorInteriorOrUpvar::Interior(interior_span, interior_extra_info) => {
2234                 if let Some((scope_span, yield_span, expr, from_awaited_ty)) = interior_extra_info {
2235                     if let Some(await_span) = from_awaited_ty {
2236                         // The type causing this obligation is one being awaited at await_span.
2237                         let mut span = MultiSpan::from_span(await_span);
2238                         span.push_span_label(
2239                             await_span,
2240                             format!(
2241                                 "await occurs here on type `{}`, which {}",
2242                                 target_ty, trait_explanation
2243                             ),
2244                         );
2245                         err.span_note(
2246                             span,
2247                             &format!(
2248                                 "future {not_trait} as it awaits another future which {not_trait}",
2249                                 not_trait = trait_explanation
2250                             ),
2251                         );
2252                     } else {
2253                         // Look at the last interior type to get a span for the `.await`.
2254                         debug!(
2255                             generator_interior_types = ?format_args!(
2256                                 "{:#?}", typeck_results.as_ref().map(|t| &t.generator_interior_types)
2257                             ),
2258                         );
2259                         explain_yield(interior_span, yield_span, scope_span);
2260                     }
2261
2262                     if let Some(expr_id) = expr {
2263                         let expr = hir.expect_expr(expr_id);
2264                         debug!("target_ty evaluated from {:?}", expr);
2265
2266                         let parent = hir.get_parent_node(expr_id);
2267                         if let Some(hir::Node::Expr(e)) = hir.find(parent) {
2268                             let parent_span = hir.span(parent);
2269                             let parent_did = parent.owner.to_def_id();
2270                             // ```rust
2271                             // impl T {
2272                             //     fn foo(&self) -> i32 {}
2273                             // }
2274                             // T.foo();
2275                             // ^^^^^^^ a temporary `&T` created inside this method call due to `&self`
2276                             // ```
2277                             //
2278                             let is_region_borrow = if let Some(typeck_results) = typeck_results {
2279                                 typeck_results
2280                                     .expr_adjustments(expr)
2281                                     .iter()
2282                                     .any(|adj| adj.is_region_borrow())
2283                             } else {
2284                                 false
2285                             };
2286
2287                             // ```rust
2288                             // struct Foo(*const u8);
2289                             // bar(Foo(std::ptr::null())).await;
2290                             //     ^^^^^^^^^^^^^^^^^^^^^ raw-ptr `*T` created inside this struct ctor.
2291                             // ```
2292                             debug!(parent_def_kind = ?self.tcx.def_kind(parent_did));
2293                             let is_raw_borrow_inside_fn_like_call =
2294                                 match self.tcx.def_kind(parent_did) {
2295                                     DefKind::Fn | DefKind::Ctor(..) => target_ty.is_unsafe_ptr(),
2296                                     _ => false,
2297                                 };
2298                             if let Some(typeck_results) = typeck_results {
2299                                 if (typeck_results.is_method_call(e) && is_region_borrow)
2300                                     || is_raw_borrow_inside_fn_like_call
2301                                 {
2302                                     err.span_help(
2303                                         parent_span,
2304                                         "consider moving this into a `let` \
2305                         binding to create a shorter lived borrow",
2306                                     );
2307                                 }
2308                             }
2309                         }
2310                     }
2311                 }
2312             }
2313             GeneratorInteriorOrUpvar::Upvar(upvar_span) => {
2314                 // `Some(ref_ty)` if `target_ty` is `&T` and `T` fails to impl `Sync`
2315                 let refers_to_non_sync = match target_ty.kind() {
2316                     ty::Ref(_, ref_ty, _) => match self.evaluate_obligation(&obligation) {
2317                         Ok(eval) if !eval.may_apply() => Some(ref_ty),
2318                         _ => None,
2319                     },
2320                     _ => None,
2321                 };
2322
2323                 let (span_label, span_note) = match refers_to_non_sync {
2324                     // if `target_ty` is `&T` and `T` fails to impl `Sync`,
2325                     // include suggestions to make `T: Sync` so that `&T: Send`
2326                     Some(ref_ty) => (
2327                         format!(
2328                             "has type `{}` which {}, because `{}` is not `Sync`",
2329                             target_ty, trait_explanation, ref_ty
2330                         ),
2331                         format!(
2332                             "captured value {} because `&` references cannot be sent unless their referent is `Sync`",
2333                             trait_explanation
2334                         ),
2335                     ),
2336                     None => (
2337                         format!("has type `{}` which {}", target_ty, trait_explanation),
2338                         format!("captured value {}", trait_explanation),
2339                     ),
2340                 };
2341
2342                 let mut span = MultiSpan::from_span(upvar_span);
2343                 span.push_span_label(upvar_span, span_label);
2344                 err.span_note(span, &span_note);
2345             }
2346         }
2347
2348         // Add a note for the item obligation that remains - normally a note pointing to the
2349         // bound that introduced the obligation (e.g. `T: Send`).
2350         debug!(?next_code);
2351         self.note_obligation_cause_code(
2352             err,
2353             &obligation.predicate,
2354             obligation.param_env,
2355             next_code.unwrap(),
2356             &mut Vec::new(),
2357             &mut Default::default(),
2358         );
2359     }
2360
2361     fn note_obligation_cause_code<T>(
2362         &self,
2363         err: &mut Diagnostic,
2364         predicate: &T,
2365         param_env: ty::ParamEnv<'tcx>,
2366         cause_code: &ObligationCauseCode<'tcx>,
2367         obligated_types: &mut Vec<Ty<'tcx>>,
2368         seen_requirements: &mut FxHashSet<DefId>,
2369     ) where
2370         T: fmt::Display,
2371     {
2372         let tcx = self.tcx;
2373         match *cause_code {
2374             ObligationCauseCode::ExprAssignable
2375             | ObligationCauseCode::MatchExpressionArm { .. }
2376             | ObligationCauseCode::Pattern { .. }
2377             | ObligationCauseCode::IfExpression { .. }
2378             | ObligationCauseCode::IfExpressionWithNoElse
2379             | ObligationCauseCode::MainFunctionType
2380             | ObligationCauseCode::StartFunctionType
2381             | ObligationCauseCode::IntrinsicType
2382             | ObligationCauseCode::MethodReceiver
2383             | ObligationCauseCode::ReturnNoExpression
2384             | ObligationCauseCode::UnifyReceiver(..)
2385             | ObligationCauseCode::OpaqueType
2386             | ObligationCauseCode::MiscObligation
2387             | ObligationCauseCode::WellFormed(..)
2388             | ObligationCauseCode::MatchImpl(..)
2389             | ObligationCauseCode::ReturnType
2390             | ObligationCauseCode::ReturnValue(_)
2391             | ObligationCauseCode::BlockTailExpression(_)
2392             | ObligationCauseCode::AwaitableExpr(_)
2393             | ObligationCauseCode::ForLoopIterator
2394             | ObligationCauseCode::QuestionMark
2395             | ObligationCauseCode::CheckAssociatedTypeBounds { .. }
2396             | ObligationCauseCode::LetElse
2397             | ObligationCauseCode::BinOp { .. }
2398             | ObligationCauseCode::AscribeUserTypeProvePredicate(..)
2399             | ObligationCauseCode::RustCall => {}
2400             ObligationCauseCode::SliceOrArrayElem => {
2401                 err.note("slice and array elements must have `Sized` type");
2402             }
2403             ObligationCauseCode::TupleElem => {
2404                 err.note("only the last element of a tuple may have a dynamically sized type");
2405             }
2406             ObligationCauseCode::ProjectionWf(data) => {
2407                 err.note(&format!("required so that the projection `{}` is well-formed", data,));
2408             }
2409             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
2410                 err.note(&format!(
2411                     "required so that reference `{}` does not outlive its referent",
2412                     ref_ty,
2413                 ));
2414             }
2415             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
2416                 err.note(&format!(
2417                     "required so that the lifetime bound of `{}` for `{}` is satisfied",
2418                     region, object_ty,
2419                 ));
2420             }
2421             ObligationCauseCode::ItemObligation(_)
2422             | ObligationCauseCode::ExprItemObligation(..) => {
2423                 // We hold the `DefId` of the item introducing the obligation, but displaying it
2424                 // doesn't add user usable information. It always point at an associated item.
2425             }
2426             ObligationCauseCode::BindingObligation(item_def_id, span)
2427             | ObligationCauseCode::ExprBindingObligation(item_def_id, span, ..) => {
2428                 let item_name = tcx.def_path_str(item_def_id);
2429                 let mut multispan = MultiSpan::from(span);
2430                 if let Some(ident) = tcx.opt_item_ident(item_def_id) {
2431                     let sm = tcx.sess.source_map();
2432                     let same_line =
2433                         match (sm.lookup_line(ident.span.hi()), sm.lookup_line(span.lo())) {
2434                             (Ok(l), Ok(r)) => l.line == r.line,
2435                             _ => true,
2436                         };
2437                     if !ident.span.is_dummy() && !ident.span.overlaps(span) && !same_line {
2438                         multispan.push_span_label(ident.span, "required by a bound in this");
2439                     }
2440                 }
2441                 let descr = format!("required by a bound in `{}`", item_name);
2442                 if !span.is_dummy() {
2443                     let msg = format!("required by this bound in `{}`", item_name);
2444                     multispan.push_span_label(span, msg);
2445                     err.span_note(multispan, &descr);
2446                 } else {
2447                     err.span_note(tcx.def_span(item_def_id), &descr);
2448                 }
2449             }
2450             ObligationCauseCode::ObjectCastObligation(concrete_ty, object_ty) => {
2451                 err.note(&format!(
2452                     "required for the cast from `{}` to the object type `{}`",
2453                     self.ty_to_string(concrete_ty),
2454                     self.ty_to_string(object_ty)
2455                 ));
2456             }
2457             ObligationCauseCode::Coercion { source: _, target } => {
2458                 err.note(&format!("required by cast to type `{}`", self.ty_to_string(target)));
2459             }
2460             ObligationCauseCode::RepeatElementCopy { is_const_fn } => {
2461                 err.note(
2462                     "the `Copy` trait is required because this value will be copied for each element of the array",
2463                 );
2464
2465                 if is_const_fn {
2466                     err.help(
2467                         "consider creating a new `const` item and initializing it with the result \
2468                         of the function call to be used in the repeat position, like \
2469                         `const VAL: Type = const_fn();` and `let x = [VAL; 42];`",
2470                     );
2471                 }
2472
2473                 if self.tcx.sess.is_nightly_build() && is_const_fn {
2474                     err.help(
2475                         "create an inline `const` block, see RFC #2920 \
2476                          <https://github.com/rust-lang/rfcs/pull/2920> for more information",
2477                     );
2478                 }
2479             }
2480             ObligationCauseCode::VariableType(hir_id) => {
2481                 let parent_node = self.tcx.hir().get_parent_node(hir_id);
2482                 match self.tcx.hir().find(parent_node) {
2483                     Some(Node::Local(hir::Local {
2484                         init: Some(hir::Expr { kind: hir::ExprKind::Index(_, _), span, .. }),
2485                         ..
2486                     })) => {
2487                         // When encountering an assignment of an unsized trait, like
2488                         // `let x = ""[..];`, provide a suggestion to borrow the initializer in
2489                         // order to use have a slice instead.
2490                         err.span_suggestion_verbose(
2491                             span.shrink_to_lo(),
2492                             "consider borrowing here",
2493                             "&",
2494                             Applicability::MachineApplicable,
2495                         );
2496                         err.note("all local variables must have a statically known size");
2497                     }
2498                     Some(Node::Param(param)) => {
2499                         err.span_suggestion_verbose(
2500                             param.ty_span.shrink_to_lo(),
2501                             "function arguments must have a statically known size, borrowed types \
2502                             always have a known size",
2503                             "&",
2504                             Applicability::MachineApplicable,
2505                         );
2506                     }
2507                     _ => {
2508                         err.note("all local variables must have a statically known size");
2509                     }
2510                 }
2511                 if !self.tcx.features().unsized_locals {
2512                     err.help("unsized locals are gated as an unstable feature");
2513                 }
2514             }
2515             ObligationCauseCode::SizedArgumentType(sp) => {
2516                 if let Some(span) = sp {
2517                     err.span_suggestion_verbose(
2518                         span.shrink_to_lo(),
2519                         "function arguments must have a statically known size, borrowed types \
2520                          always have a known size",
2521                         "&",
2522                         Applicability::MachineApplicable,
2523                     );
2524                 } else {
2525                     err.note("all function arguments must have a statically known size");
2526                 }
2527                 if tcx.sess.opts.unstable_features.is_nightly_build()
2528                     && !self.tcx.features().unsized_fn_params
2529                 {
2530                     err.help("unsized fn params are gated as an unstable feature");
2531                 }
2532             }
2533             ObligationCauseCode::SizedReturnType => {
2534                 err.note("the return type of a function must have a statically known size");
2535             }
2536             ObligationCauseCode::SizedYieldType => {
2537                 err.note("the yield type of a generator must have a statically known size");
2538             }
2539             ObligationCauseCode::SizedBoxType => {
2540                 err.note("the type of a box expression must have a statically known size");
2541             }
2542             ObligationCauseCode::AssignmentLhsSized => {
2543                 err.note("the left-hand-side of an assignment must have a statically known size");
2544             }
2545             ObligationCauseCode::TupleInitializerSized => {
2546                 err.note("tuples must have a statically known size to be initialized");
2547             }
2548             ObligationCauseCode::StructInitializerSized => {
2549                 err.note("structs must have a statically known size to be initialized");
2550             }
2551             ObligationCauseCode::FieldSized { adt_kind: ref item, last, span } => {
2552                 match *item {
2553                     AdtKind::Struct => {
2554                         if last {
2555                             err.note(
2556                                 "the last field of a packed struct may only have a \
2557                                 dynamically sized type if it does not need drop to be run",
2558                             );
2559                         } else {
2560                             err.note(
2561                                 "only the last field of a struct may have a dynamically sized type",
2562                             );
2563                         }
2564                     }
2565                     AdtKind::Union => {
2566                         err.note("no field of a union may have a dynamically sized type");
2567                     }
2568                     AdtKind::Enum => {
2569                         err.note("no field of an enum variant may have a dynamically sized type");
2570                     }
2571                 }
2572                 err.help("change the field's type to have a statically known size");
2573                 err.span_suggestion(
2574                     span.shrink_to_lo(),
2575                     "borrowed types always have a statically known size",
2576                     "&",
2577                     Applicability::MachineApplicable,
2578                 );
2579                 err.multipart_suggestion(
2580                     "the `Box` type always has a statically known size and allocates its contents \
2581                      in the heap",
2582                     vec![
2583                         (span.shrink_to_lo(), "Box<".to_string()),
2584                         (span.shrink_to_hi(), ">".to_string()),
2585                     ],
2586                     Applicability::MachineApplicable,
2587                 );
2588             }
2589             ObligationCauseCode::ConstSized => {
2590                 err.note("constant expressions must have a statically known size");
2591             }
2592             ObligationCauseCode::InlineAsmSized => {
2593                 err.note("all inline asm arguments must have a statically known size");
2594             }
2595             ObligationCauseCode::ConstPatternStructural => {
2596                 err.note("constants used for pattern-matching must derive `PartialEq` and `Eq`");
2597             }
2598             ObligationCauseCode::SharedStatic => {
2599                 err.note("shared static variables must have a type that implements `Sync`");
2600             }
2601             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
2602                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2603                 let ty = parent_trait_ref.skip_binder().self_ty();
2604                 if parent_trait_ref.references_error() {
2605                     // NOTE(eddyb) this was `.cancel()`, but `err`
2606                     // is borrowed, so we can't fully defuse it.
2607                     err.downgrade_to_delayed_bug();
2608                     return;
2609                 }
2610
2611                 // If the obligation for a tuple is set directly by a Generator or Closure,
2612                 // then the tuple must be the one containing capture types.
2613                 let is_upvar_tys_infer_tuple = if !matches!(ty.kind(), ty::Tuple(..)) {
2614                     false
2615                 } else {
2616                     if let ObligationCauseCode::BuiltinDerivedObligation(data) = &*data.parent_code
2617                     {
2618                         let parent_trait_ref =
2619                             self.resolve_vars_if_possible(data.parent_trait_pred);
2620                         let nested_ty = parent_trait_ref.skip_binder().self_ty();
2621                         matches!(nested_ty.kind(), ty::Generator(..))
2622                             || matches!(nested_ty.kind(), ty::Closure(..))
2623                     } else {
2624                         false
2625                     }
2626                 };
2627
2628                 let from_generator = tcx.lang_items().from_generator_fn().unwrap();
2629
2630                 // Don't print the tuple of capture types
2631                 'print: {
2632                     if !is_upvar_tys_infer_tuple {
2633                         let msg = format!("required because it appears within the type `{}`", ty);
2634                         match ty.kind() {
2635                             ty::Adt(def, _) => {
2636                                 // `gen_future` is used in all async functions; it doesn't add any additional info.
2637                                 if self.tcx.is_diagnostic_item(sym::gen_future, def.did()) {
2638                                     break 'print;
2639                                 }
2640                                 match self.tcx.opt_item_ident(def.did()) {
2641                                     Some(ident) => err.span_note(ident.span, &msg),
2642                                     None => err.note(&msg),
2643                                 }
2644                             }
2645                             ty::Opaque(def_id, _) => {
2646                                 // Avoid printing the future from `core::future::from_generator`, it's not helpful
2647                                 if tcx.parent(*def_id) == from_generator {
2648                                     break 'print;
2649                                 }
2650
2651                                 // If the previous type is `from_generator`, this is the future generated by the body of an async function.
2652                                 // Avoid printing it twice (it was already printed in the `ty::Generator` arm below).
2653                                 let is_future = tcx.ty_is_opaque_future(ty);
2654                                 debug!(
2655                                     ?obligated_types,
2656                                     ?is_future,
2657                                     "note_obligation_cause_code: check for async fn"
2658                                 );
2659                                 if is_future
2660                                     && obligated_types.last().map_or(false, |ty| match ty.kind() {
2661                                         ty::Opaque(last_def_id, _) => {
2662                                             tcx.parent(*last_def_id) == from_generator
2663                                         }
2664                                         _ => false,
2665                                     })
2666                                 {
2667                                     break 'print;
2668                                 }
2669                                 err.span_note(self.tcx.def_span(def_id), &msg)
2670                             }
2671                             ty::GeneratorWitness(bound_tys) => {
2672                                 use std::fmt::Write;
2673
2674                                 // FIXME: this is kind of an unusual format for rustc, can we make it more clear?
2675                                 // Maybe we should just remove this note altogether?
2676                                 // FIXME: only print types which don't meet the trait requirement
2677                                 let mut msg =
2678                                     "required because it captures the following types: ".to_owned();
2679                                 for ty in bound_tys.skip_binder() {
2680                                     write!(msg, "`{}`, ", ty).unwrap();
2681                                 }
2682                                 err.note(msg.trim_end_matches(", "))
2683                             }
2684                             ty::Generator(def_id, _, _) => {
2685                                 let sp = self.tcx.def_span(def_id);
2686
2687                                 // Special-case this to say "async block" instead of `[static generator]`.
2688                                 let kind = tcx.generator_kind(def_id).unwrap();
2689                                 err.span_note(
2690                                     sp,
2691                                     &format!("required because it's used within this {}", kind),
2692                                 )
2693                             }
2694                             ty::Closure(def_id, _) => err.span_note(
2695                                 self.tcx.def_span(def_id),
2696                                 &format!("required because it's used within this closure"),
2697                             ),
2698                             _ => err.note(&msg),
2699                         };
2700                     }
2701                 }
2702
2703                 obligated_types.push(ty);
2704
2705                 let parent_predicate = parent_trait_ref;
2706                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
2707                     // #74711: avoid a stack overflow
2708                     ensure_sufficient_stack(|| {
2709                         self.note_obligation_cause_code(
2710                             err,
2711                             &parent_predicate,
2712                             param_env,
2713                             &data.parent_code,
2714                             obligated_types,
2715                             seen_requirements,
2716                         )
2717                     });
2718                 } else {
2719                     ensure_sufficient_stack(|| {
2720                         self.note_obligation_cause_code(
2721                             err,
2722                             &parent_predicate,
2723                             param_env,
2724                             cause_code.peel_derives(),
2725                             obligated_types,
2726                             seen_requirements,
2727                         )
2728                     });
2729                 }
2730             }
2731             ObligationCauseCode::ImplDerivedObligation(ref data) => {
2732                 let mut parent_trait_pred =
2733                     self.resolve_vars_if_possible(data.derived.parent_trait_pred);
2734                 parent_trait_pred.remap_constness_diag(param_env);
2735                 let parent_def_id = parent_trait_pred.def_id();
2736                 let msg = format!(
2737                     "required for `{}` to implement `{}`",
2738                     parent_trait_pred.skip_binder().self_ty(),
2739                     parent_trait_pred.print_modifiers_and_trait_path()
2740                 );
2741                 let mut is_auto_trait = false;
2742                 match self.tcx.hir().get_if_local(data.impl_def_id) {
2743                     Some(Node::Item(hir::Item {
2744                         kind: hir::ItemKind::Trait(is_auto, ..),
2745                         ident,
2746                         ..
2747                     })) => {
2748                         // FIXME: we should do something else so that it works even on crate foreign
2749                         // auto traits.
2750                         is_auto_trait = matches!(is_auto, hir::IsAuto::Yes);
2751                         err.span_note(ident.span, &msg)
2752                     }
2753                     Some(Node::Item(hir::Item {
2754                         kind: hir::ItemKind::Impl(hir::Impl { of_trait, self_ty, .. }),
2755                         ..
2756                     })) => {
2757                         let mut spans = Vec::with_capacity(2);
2758                         if let Some(trait_ref) = of_trait {
2759                             spans.push(trait_ref.path.span);
2760                         }
2761                         spans.push(self_ty.span);
2762                         err.span_note(spans, &msg)
2763                     }
2764                     _ => err.note(&msg),
2765                 };
2766
2767                 let mut parent_predicate = parent_trait_pred;
2768                 let mut data = &data.derived;
2769                 let mut count = 0;
2770                 seen_requirements.insert(parent_def_id);
2771                 if is_auto_trait {
2772                     // We don't want to point at the ADT saying "required because it appears within
2773                     // the type `X`", like we would otherwise do in test `supertrait-auto-trait.rs`.
2774                     while let ObligationCauseCode::BuiltinDerivedObligation(derived) =
2775                         &*data.parent_code
2776                     {
2777                         let child_trait_ref =
2778                             self.resolve_vars_if_possible(derived.parent_trait_pred);
2779                         let child_def_id = child_trait_ref.def_id();
2780                         if seen_requirements.insert(child_def_id) {
2781                             break;
2782                         }
2783                         data = derived;
2784                         parent_predicate = child_trait_ref.to_predicate(tcx);
2785                         parent_trait_pred = child_trait_ref;
2786                     }
2787                 }
2788                 while let ObligationCauseCode::ImplDerivedObligation(child) = &*data.parent_code {
2789                     // Skip redundant recursive obligation notes. See `ui/issue-20413.rs`.
2790                     let child_trait_pred =
2791                         self.resolve_vars_if_possible(child.derived.parent_trait_pred);
2792                     let child_def_id = child_trait_pred.def_id();
2793                     if seen_requirements.insert(child_def_id) {
2794                         break;
2795                     }
2796                     count += 1;
2797                     data = &child.derived;
2798                     parent_predicate = child_trait_pred.to_predicate(tcx);
2799                     parent_trait_pred = child_trait_pred;
2800                 }
2801                 if count > 0 {
2802                     err.note(&format!(
2803                         "{} redundant requirement{} hidden",
2804                         count,
2805                         pluralize!(count)
2806                     ));
2807                     err.note(&format!(
2808                         "required for `{}` to implement `{}`",
2809                         parent_trait_pred.skip_binder().self_ty(),
2810                         parent_trait_pred.print_modifiers_and_trait_path()
2811                     ));
2812                 }
2813                 // #74711: avoid a stack overflow
2814                 ensure_sufficient_stack(|| {
2815                     self.note_obligation_cause_code(
2816                         err,
2817                         &parent_predicate,
2818                         param_env,
2819                         &data.parent_code,
2820                         obligated_types,
2821                         seen_requirements,
2822                     )
2823                 });
2824             }
2825             ObligationCauseCode::DerivedObligation(ref data) => {
2826                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2827                 let parent_predicate = parent_trait_ref;
2828                 // #74711: avoid a stack overflow
2829                 ensure_sufficient_stack(|| {
2830                     self.note_obligation_cause_code(
2831                         err,
2832                         &parent_predicate,
2833                         param_env,
2834                         &data.parent_code,
2835                         obligated_types,
2836                         seen_requirements,
2837                     )
2838                 });
2839             }
2840             ObligationCauseCode::FunctionArgumentObligation {
2841                 arg_hir_id,
2842                 call_hir_id,
2843                 ref parent_code,
2844             } => {
2845                 let hir = self.tcx.hir();
2846                 if let Some(Node::Expr(expr @ hir::Expr { kind: hir::ExprKind::Block(..), .. })) =
2847                     hir.find(arg_hir_id)
2848                 {
2849                     let parent_id = hir.get_parent_item(arg_hir_id);
2850                     let typeck_results: &TypeckResults<'tcx> = match &self.typeck_results {
2851                         Some(t) if t.hir_owner == parent_id => t,
2852                         _ => self.tcx.typeck(parent_id.def_id),
2853                     };
2854                     let expr = expr.peel_blocks();
2855                     let ty = typeck_results.expr_ty_adjusted_opt(expr).unwrap_or(tcx.ty_error());
2856                     let span = expr.span;
2857                     if Some(span) != err.span.primary_span() {
2858                         err.span_label(
2859                             span,
2860                             if ty.references_error() {
2861                                 String::new()
2862                             } else {
2863                                 format!("this tail expression is of type `{:?}`", ty)
2864                             },
2865                         );
2866                     }
2867                 }
2868                 if let Some(Node::Expr(hir::Expr {
2869                     kind:
2870                         hir::ExprKind::Call(hir::Expr { span, .. }, _)
2871                         | hir::ExprKind::MethodCall(
2872                             hir::PathSegment { ident: Ident { span, .. }, .. },
2873                             ..,
2874                         ),
2875                     ..
2876                 })) = hir.find(call_hir_id)
2877                 {
2878                     if Some(*span) != err.span.primary_span() {
2879                         err.span_label(*span, "required by a bound introduced by this call");
2880                     }
2881                 }
2882                 ensure_sufficient_stack(|| {
2883                     self.note_obligation_cause_code(
2884                         err,
2885                         predicate,
2886                         param_env,
2887                         &parent_code,
2888                         obligated_types,
2889                         seen_requirements,
2890                     )
2891                 });
2892             }
2893             ObligationCauseCode::CompareImplItemObligation { trait_item_def_id, kind, .. } => {
2894                 let item_name = self.tcx.item_name(trait_item_def_id);
2895                 let msg = format!(
2896                     "the requirement `{}` appears on the `impl`'s {kind} `{}` but not on the \
2897                      corresponding trait's {kind}",
2898                     predicate, item_name,
2899                 );
2900                 let sp = self
2901                     .tcx
2902                     .opt_item_ident(trait_item_def_id)
2903                     .map(|i| i.span)
2904                     .unwrap_or_else(|| self.tcx.def_span(trait_item_def_id));
2905                 let mut assoc_span: MultiSpan = sp.into();
2906                 assoc_span.push_span_label(
2907                     sp,
2908                     format!("this trait's {kind} doesn't have the requirement `{}`", predicate),
2909                 );
2910                 if let Some(ident) = self
2911                     .tcx
2912                     .opt_associated_item(trait_item_def_id)
2913                     .and_then(|i| self.tcx.opt_item_ident(i.container_id(self.tcx)))
2914                 {
2915                     assoc_span.push_span_label(ident.span, "in this trait");
2916                 }
2917                 err.span_note(assoc_span, &msg);
2918             }
2919             ObligationCauseCode::TrivialBound => {
2920                 err.help("see issue #48214");
2921                 if tcx.sess.opts.unstable_features.is_nightly_build() {
2922                     err.help("add `#![feature(trivial_bounds)]` to the crate attributes to enable");
2923                 }
2924             }
2925             ObligationCauseCode::OpaqueReturnType(expr_info) => {
2926                 if let Some((expr_ty, expr_span)) = expr_info {
2927                     let expr_ty = self.resolve_vars_if_possible(expr_ty);
2928                     err.span_label(
2929                         expr_span,
2930                         format!("return type was inferred to be `{expr_ty}` here"),
2931                     );
2932                 }
2933             }
2934         }
2935     }
2936
2937     #[instrument(
2938         level = "debug", skip(self, err), fields(trait_pred.self_ty = ?trait_pred.self_ty())
2939     )]
2940     fn suggest_await_before_try(
2941         &self,
2942         err: &mut Diagnostic,
2943         obligation: &PredicateObligation<'tcx>,
2944         trait_pred: ty::PolyTraitPredicate<'tcx>,
2945         span: Span,
2946     ) {
2947         let body_hir_id = obligation.cause.body_id;
2948         let item_id = self.tcx.hir().get_parent_node(body_hir_id);
2949
2950         if let Some(body_id) =
2951             self.tcx.hir().maybe_body_owned_by(self.tcx.hir().local_def_id(item_id))
2952         {
2953             let body = self.tcx.hir().body(body_id);
2954             if let Some(hir::GeneratorKind::Async(_)) = body.generator_kind {
2955                 let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
2956
2957                 let self_ty = self.resolve_vars_if_possible(trait_pred.self_ty());
2958                 let impls_future = self.type_implements_trait(
2959                     future_trait,
2960                     self.tcx.erase_late_bound_regions(self_ty),
2961                     ty::List::empty(),
2962                     obligation.param_env,
2963                 );
2964                 if !impls_future.must_apply_modulo_regions() {
2965                     return;
2966                 }
2967
2968                 let item_def_id = self.tcx.associated_item_def_ids(future_trait)[0];
2969                 // `<T as Future>::Output`
2970                 let projection_ty = trait_pred.map_bound(|trait_pred| {
2971                     self.tcx.mk_projection(
2972                         item_def_id,
2973                         // Future::Output has no substs
2974                         self.tcx.mk_substs_trait(trait_pred.self_ty(), &[]),
2975                     )
2976                 });
2977                 let projection_ty = normalize_to(
2978                     &mut SelectionContext::new(self),
2979                     obligation.param_env,
2980                     obligation.cause.clone(),
2981                     projection_ty,
2982                     &mut vec![],
2983                 );
2984
2985                 debug!(
2986                     normalized_projection_type = ?self.resolve_vars_if_possible(projection_ty)
2987                 );
2988                 let try_obligation = self.mk_trait_obligation_with_new_self_ty(
2989                     obligation.param_env,
2990                     trait_pred.map_bound(|trait_pred| (trait_pred, projection_ty.skip_binder())),
2991                 );
2992                 debug!(try_trait_obligation = ?try_obligation);
2993                 if self.predicate_may_hold(&try_obligation)
2994                     && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
2995                     && snippet.ends_with('?')
2996                 {
2997                     err.span_suggestion_verbose(
2998                         span.with_hi(span.hi() - BytePos(1)).shrink_to_hi(),
2999                         "consider `await`ing on the `Future`",
3000                         ".await",
3001                         Applicability::MaybeIncorrect,
3002                     );
3003                 }
3004             }
3005         }
3006     }
3007
3008     fn suggest_floating_point_literal(
3009         &self,
3010         obligation: &PredicateObligation<'tcx>,
3011         err: &mut Diagnostic,
3012         trait_ref: &ty::PolyTraitRef<'tcx>,
3013     ) {
3014         let rhs_span = match obligation.cause.code() {
3015             ObligationCauseCode::BinOp { rhs_span: Some(span), is_lit, .. } if *is_lit => span,
3016             _ => return,
3017         };
3018         if let ty::Float(_) = trait_ref.skip_binder().self_ty().kind()
3019             && let ty::Infer(InferTy::IntVar(_)) = trait_ref.skip_binder().substs.type_at(1).kind()
3020         {
3021             err.span_suggestion_verbose(
3022                 rhs_span.shrink_to_hi(),
3023                 "consider using a floating-point literal by writing it with `.0`",
3024                 ".0",
3025                 Applicability::MaybeIncorrect,
3026             );
3027         }
3028     }
3029
3030     fn suggest_derive(
3031         &self,
3032         obligation: &PredicateObligation<'tcx>,
3033         err: &mut Diagnostic,
3034         trait_pred: ty::PolyTraitPredicate<'tcx>,
3035     ) {
3036         let Some(diagnostic_name) = self.tcx.get_diagnostic_name(trait_pred.def_id()) else {
3037             return;
3038         };
3039         let (adt, substs) = match trait_pred.skip_binder().self_ty().kind() {
3040             ty::Adt(adt, substs) if adt.did().is_local() => (adt, substs),
3041             _ => return,
3042         };
3043         let can_derive = {
3044             let is_derivable_trait = match diagnostic_name {
3045                 sym::Default => !adt.is_enum(),
3046                 sym::PartialEq | sym::PartialOrd => {
3047                     let rhs_ty = trait_pred.skip_binder().trait_ref.substs.type_at(1);
3048                     trait_pred.skip_binder().self_ty() == rhs_ty
3049                 }
3050                 sym::Eq | sym::Ord | sym::Clone | sym::Copy | sym::Hash | sym::Debug => true,
3051                 _ => false,
3052             };
3053             is_derivable_trait &&
3054                 // Ensure all fields impl the trait.
3055                 adt.all_fields().all(|field| {
3056                     let field_ty = field.ty(self.tcx, substs);
3057                     let trait_substs = match diagnostic_name {
3058                         sym::PartialEq | sym::PartialOrd => {
3059                             self.tcx.mk_substs_trait(field_ty, &[field_ty.into()])
3060                         }
3061                         _ => self.tcx.mk_substs_trait(field_ty, &[]),
3062                     };
3063                     let trait_pred = trait_pred.map_bound_ref(|tr| ty::TraitPredicate {
3064                         trait_ref: ty::TraitRef {
3065                             substs: trait_substs,
3066                             ..trait_pred.skip_binder().trait_ref
3067                         },
3068                         ..*tr
3069                     });
3070                     let field_obl = Obligation::new(
3071                         self.tcx,
3072                         obligation.cause.clone(),
3073                         obligation.param_env,
3074                         trait_pred,
3075                     );
3076                     self.predicate_must_hold_modulo_regions(&field_obl)
3077                 })
3078         };
3079         if can_derive {
3080             err.span_suggestion_verbose(
3081                 self.tcx.def_span(adt.did()).shrink_to_lo(),
3082                 &format!(
3083                     "consider annotating `{}` with `#[derive({})]`",
3084                     trait_pred.skip_binder().self_ty(),
3085                     diagnostic_name,
3086                 ),
3087                 format!("#[derive({})]\n", diagnostic_name),
3088                 Applicability::MaybeIncorrect,
3089             );
3090         }
3091     }
3092
3093     fn suggest_dereferencing_index(
3094         &self,
3095         obligation: &PredicateObligation<'tcx>,
3096         err: &mut Diagnostic,
3097         trait_pred: ty::PolyTraitPredicate<'tcx>,
3098     ) {
3099         if let ObligationCauseCode::ImplDerivedObligation(_) = obligation.cause.code()
3100             && self.tcx.is_diagnostic_item(sym::SliceIndex, trait_pred.skip_binder().trait_ref.def_id)
3101             && let ty::Slice(_) = trait_pred.skip_binder().trait_ref.substs.type_at(1).kind()
3102             && let ty::Ref(_, inner_ty, _) = trait_pred.skip_binder().self_ty().kind()
3103             && let ty::Uint(ty::UintTy::Usize) = inner_ty.kind()
3104         {
3105             err.span_suggestion_verbose(
3106                 obligation.cause.span.shrink_to_lo(),
3107             "dereference this index",
3108             '*',
3109                 Applicability::MachineApplicable,
3110             );
3111         }
3112     }
3113 }
3114
3115 /// Collect all the returned expressions within the input expression.
3116 /// Used to point at the return spans when we want to suggest some change to them.
3117 #[derive(Default)]
3118 pub struct ReturnsVisitor<'v> {
3119     pub returns: Vec<&'v hir::Expr<'v>>,
3120     in_block_tail: bool,
3121 }
3122
3123 impl<'v> Visitor<'v> for ReturnsVisitor<'v> {
3124     fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
3125         // Visit every expression to detect `return` paths, either through the function's tail
3126         // expression or `return` statements. We walk all nodes to find `return` statements, but
3127         // we only care about tail expressions when `in_block_tail` is `true`, which means that
3128         // they're in the return path of the function body.
3129         match ex.kind {
3130             hir::ExprKind::Ret(Some(ex)) => {
3131                 self.returns.push(ex);
3132             }
3133             hir::ExprKind::Block(block, _) if self.in_block_tail => {
3134                 self.in_block_tail = false;
3135                 for stmt in block.stmts {
3136                     hir::intravisit::walk_stmt(self, stmt);
3137                 }
3138                 self.in_block_tail = true;
3139                 if let Some(expr) = block.expr {
3140                     self.visit_expr(expr);
3141                 }
3142             }
3143             hir::ExprKind::If(_, then, else_opt) if self.in_block_tail => {
3144                 self.visit_expr(then);
3145                 if let Some(el) = else_opt {
3146                     self.visit_expr(el);
3147                 }
3148             }
3149             hir::ExprKind::Match(_, arms, _) if self.in_block_tail => {
3150                 for arm in arms {
3151                     self.visit_expr(arm.body);
3152                 }
3153             }
3154             // We need to walk to find `return`s in the entire body.
3155             _ if !self.in_block_tail => hir::intravisit::walk_expr(self, ex),
3156             _ => self.returns.push(ex),
3157         }
3158     }
3159
3160     fn visit_body(&mut self, body: &'v hir::Body<'v>) {
3161         assert!(!self.in_block_tail);
3162         if body.generator_kind().is_none() {
3163             if let hir::ExprKind::Block(block, None) = body.value.kind {
3164                 if block.expr.is_some() {
3165                     self.in_block_tail = true;
3166                 }
3167             }
3168         }
3169         hir::intravisit::walk_body(self, body);
3170     }
3171 }
3172
3173 /// Collect all the awaited expressions within the input expression.
3174 #[derive(Default)]
3175 struct AwaitsVisitor {
3176     awaits: Vec<hir::HirId>,
3177 }
3178
3179 impl<'v> Visitor<'v> for AwaitsVisitor {
3180     fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
3181         if let hir::ExprKind::Yield(_, hir::YieldSource::Await { expr: Some(id) }) = ex.kind {
3182             self.awaits.push(id)
3183         }
3184         hir::intravisit::walk_expr(self, ex)
3185     }
3186 }
3187
3188 pub trait NextTypeParamName {
3189     fn next_type_param_name(&self, name: Option<&str>) -> String;
3190 }
3191
3192 impl NextTypeParamName for &[hir::GenericParam<'_>] {
3193     fn next_type_param_name(&self, name: Option<&str>) -> String {
3194         // This is the list of possible parameter names that we might suggest.
3195         let name = name.and_then(|n| n.chars().next()).map(|c| c.to_string().to_uppercase());
3196         let name = name.as_deref();
3197         let possible_names = [name.unwrap_or("T"), "T", "U", "V", "X", "Y", "Z", "A", "B", "C"];
3198         let used_names = self
3199             .iter()
3200             .filter_map(|p| match p.name {
3201                 hir::ParamName::Plain(ident) => Some(ident.name),
3202                 _ => None,
3203             })
3204             .collect::<Vec<_>>();
3205
3206         possible_names
3207             .iter()
3208             .find(|n| !used_names.contains(&Symbol::intern(n)))
3209             .unwrap_or(&"ParamName")
3210             .to_string()
3211     }
3212 }
3213
3214 fn suggest_trait_object_return_type_alternatives(
3215     err: &mut Diagnostic,
3216     ret_ty: Span,
3217     trait_obj: &str,
3218     is_object_safe: bool,
3219 ) {
3220     err.span_suggestion(
3221         ret_ty,
3222         "use some type `T` that is `T: Sized` as the return type if all return paths have the \
3223             same type",
3224         "T",
3225         Applicability::MaybeIncorrect,
3226     );
3227     err.span_suggestion(
3228         ret_ty,
3229         &format!(
3230             "use `impl {}` as the return type if all return paths have the same type but you \
3231                 want to expose only the trait in the signature",
3232             trait_obj,
3233         ),
3234         format!("impl {}", trait_obj),
3235         Applicability::MaybeIncorrect,
3236     );
3237     if is_object_safe {
3238         err.multipart_suggestion(
3239             &format!(
3240                 "use a boxed trait object if all return paths implement trait `{}`",
3241                 trait_obj,
3242             ),
3243             vec![
3244                 (ret_ty.shrink_to_lo(), "Box<".to_string()),
3245                 (ret_ty.shrink_to_hi(), ">".to_string()),
3246             ],
3247             Applicability::MaybeIncorrect,
3248         );
3249     }
3250 }
3251
3252 /// Collect the spans that we see the generic param `param_did`
3253 struct ReplaceImplTraitVisitor<'a> {
3254     ty_spans: &'a mut Vec<Span>,
3255     param_did: DefId,
3256 }
3257
3258 impl<'a, 'hir> hir::intravisit::Visitor<'hir> for ReplaceImplTraitVisitor<'a> {
3259     fn visit_ty(&mut self, t: &'hir hir::Ty<'hir>) {
3260         if let hir::TyKind::Path(hir::QPath::Resolved(
3261             None,
3262             hir::Path { res: hir::def::Res::Def(_, segment_did), .. },
3263         )) = t.kind
3264         {
3265             if self.param_did == *segment_did {
3266                 // `fn foo(t: impl Trait)`
3267                 //            ^^^^^^^^^^ get this to suggest `T` instead
3268
3269                 // There might be more than one `impl Trait`.
3270                 self.ty_spans.push(t.span);
3271                 return;
3272             }
3273         }
3274
3275         hir::intravisit::walk_ty(self, t);
3276     }
3277 }
3278
3279 // Replace `param` with `replace_ty`
3280 struct ReplaceImplTraitFolder<'tcx> {
3281     tcx: TyCtxt<'tcx>,
3282     param: &'tcx ty::GenericParamDef,
3283     replace_ty: Ty<'tcx>,
3284 }
3285
3286 impl<'tcx> TypeFolder<'tcx> for ReplaceImplTraitFolder<'tcx> {
3287     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
3288         if let ty::Param(ty::ParamTy { index, .. }) = t.kind() {
3289             if self.param.index == *index {
3290                 return self.replace_ty;
3291             }
3292         }
3293         t.super_fold_with(self)
3294     }
3295
3296     fn tcx(&self) -> TyCtxt<'tcx> {
3297         self.tcx
3298     }
3299 }